Rishi Sunak was warned on Monday not to trade off the power to overrule the EU in Northern Ireland in order to strike a deal with Brussels as he vowed to forge ahead with post-Brexit reforms.
Two former UK negotiators said the Prime Minister must press ahead with a new law to give the Government the right to rip up EU red tape.
Lord Frost, who led the trade talks with Brussels, told The Telegraph the Government should drive through the Protocol Bill even if a pact with the bloc was struck.
Hugh Bennett, who served on Lord Frost's team and was a special adviser to Boris Johnson and Liz Truss, added that dropping the Bill would be "a major strategic mistake".
The pair issued their plea amid reports that British and EU negotiators are closing in on a deal to end the standoff over the Northern Ireland Protocol. Both sides are keen to reach an agreement in time for the 25th anniversary of the Good Friday Agreement in April.
It came as Mr Sunak marked Tuesday's third anniversary of Brexit by insisting the reforms passed so far were "just the beginning", saying: "Whether leading Europe's fastest vaccine rollout, striking trade deals with over 70 countries or taking back control of our borders, we've forged a path as an independent nation with confidence.
"I'm determined to ensure the benefits of Brexit continue to empower communities and businesses right across the country."
Tory eurosceptics fear Mr Sunak will agree to drop the Protocol Bill, currently stuck in the Lords, in return for a deal with the EU. The legislation would give ministers the power to override large parts of the border pact but has been put on pause while talks with Brussels are ongoing.
Its introduction last June prompted a furious reaction from the EU, which has demanded it be dropped before signing up to any agreement.
Lord Frost said: "It's entirely reasonable for the Government to see if it can reach a worthwhile deal with the EU over Northern Ireland, but it has already weakened its hand to get one by halting the Protocol Bill.
"So it's crucial it doesn't abandon the powers in the Bill altogether, in case it can't get a deal or in case a deal comes apart under pressure later - which is all too likely given the EU's obtuseness over Northern Ireland so far."
Eurosceptic MPs fear the law will be shorn of its powers and turned into a piece of "implementation" legislation for the new agreement.
Mr Bennett said doing so would leave the Government without any "leverage" over Brussels if future problems arise, adding: "The United Kingdom must ask itself frankly what it is securing in return if the price demanded is to concede on its fundamental ability to govern within its own borders.
"That is why it is so important that the UK must not agree to pause, drop or hollow out the Northern Ireland Protocol Bill as a condition of any deal with the EU, or turn it into a simple implementation Bill for a deal.
"To do so risks being a major strategic mistake for years to come. Without the Bill, UK ministers will neither have the leverage to bring the EU back to the table nor the power to make anything beyond the most trivial changes themselves."
David Jones, a former Brexit minister and the deputy chairman of the European Research Group, said: "The only time the EU actually responds is when you wave a big stick.
"The Government should be pressing on with the Protocol Bill as a matter of priority to see that Northern Ireland's semi-detached status to the rest of the country is put to an end as soon as possible."
The warning comes after Archie Norman, the Marks and Spencer chairman, called plans to ease post-Brexit trade between Great Britain and Northern Ireland "baffling" and "overbearing".
A Foreign Office spokesman said: "The UK's priority is protecting the Belfast [Good Friday] Agreement and preserving political stability in Northern Ireland and the UK internal market. The Government is currently engaging in intensive scoping talks with the EU to find solutions to these problems."
By Hugh Bennett
The UK and the EU are once again approaching that familiar point in the post-Brexit diplomatic cycle where positive murmurings start emanating from London, Brussels and Dublin about a possible deal to fix the problems of the Northern Ireland Protocol.
This is, of course, grounds for optimism. Finding a lasting resolution is a challenge which has already outlasted three Prime Ministers - even ending the career of one. Meanwhile, people and businesses in Northern Ireland continue to face increased costs, reduced consumer choice and political uncertainty, straining the political institutions of the Belfast (Good Friday) Agreement. Anything which eases these pressures will be a welcome development.
However, it is also grounds for caution. False dawns have flickered before, only for the gaps to once again prove unbridgeable. The stubbornness of the problems attests to the fact that easy answers which make all sides equally happy do not exist. Solutions look workable on paper, only not to survive contact with reality, particularly against the complex backdrop of Northern Ireland society. Any deal which looks too good to be true is likely to be just that.
That is not to say that a deal that genuinely moves the dial is a hopeless proposition - far from it. Both the UK and the EU have a key advantage now in that rather than having to negotiate an unprecedented deal in a purely theoretical space, they now have the advantage of two years' practical experience of operating the Protocol, which has taught a number of key lessons.
Firstly, the evidence shows that real-world risks to the EU single market are vanishingly small. Even with the Protocol only partially implemented due to the ongoing grace periods, any evidence of rogue products 'leaking' into the EU is virtually non-existent. Rather than defaulting to a theological approach to managing risk, a genuinely bespoke approach based on empirical observation, not rigid legalism, can clearly work - provided both sides are willing.
Secondly, it is clear that 'out-of-the-box' models based on existing EU processes and legal structures are not sustainable long-term solutions for the specific circumstances of Northern Ireland. EU customs procedures may work adequately for bulk shipments from China arriving by the container-load in Rotterdam, but for small mixed shipments crossing the Irish Sea every day, the bureaucratic demands rapidly spiral out of all proportion.
Thirdly, both sides must have their eyes open to the fact that any deal now is very unlikely to be the final word on the issue, even if it does mark a significant step forward. No deal will get everything right at first attempt, nor solve every issue conclusively. If they are committed to making things work, they must accept that this will mean an ongoing process of learning by doing, talking with open minds and making further changes as future challenges arise.
New negotiations and new agreements are a perennial feature of Northern Ireland's modern political landscape and will invariably remain so if the Belfast (Good Friday) Agreement and its institutions are to function effectively into the future. Overselling a positive but inherently imperfect deal as a conclusive breakthrough risks prolonging the political crisis in Northern Ireland, rather than helping to resolve it.
Rishi Sunak has demonstrated his tenacity and strategic nous in persevering with his predecessors' strategy thus far, resisting the temptation to seek a quick win ahead of the twenty-fifth anniversary of the Agreement, or simply to clear a thorny domestic and international issue off the table.
A diplomatic 'reset' of relations with the EU may have superficial appeal within the corridors of Whitehall, but has limited intrinsic value of its own in the absence of significant policy changes elsewhere in the relationship. The United Kingdom must ask itself frankly what it is securing in return if the price demanded is to concede on its fundamental ability to govern within its own borders.
As its sovereign government, the greatest responsibility for peace and stability in Northern Ireland and the preservation of the Belfast (Good Friday) Agreement will inevitably fall on the UK Government in practice.
That is why it is so important that the UK must not agree to pause, drop or hollow out the Northern Ireland Protocol Bill as a condition of any deal with the EU, or turn it into a simple implementation bill for a deal. The Government would be surrendering its future means to act decisively to protect that stability, should exceptional circumstances demand it. To do so risks being a major strategic mistake for years to come.
Any deal will inevitably need to be refined in the future as its practical effects become better understood. Without the Bill, UK ministers will neither have the leverage to bring the EU back to the table, nor the power to make anything beyond the most trivial changes themselves, due to the way the Protocol is implemented via the EU doctrine of 'direct effect' into UK domestic law.
Many fundamental areas of the Protocol remain proverbial dogs which have yet to bark: state aid, regulatory divergence and the EU Court of Justice, to name a few. These are all issues which concern basic functions of a self-governing state - its ability to set taxes, determine public spending and pass laws which are judged in its own courts.
The EU have never seen these as their problems as their effects are largely felt only behind closed doors. Whitehall simply tells ministers they cannot enact certain policies and must self-censor their ambitions, or they are struck out preemptively before they even reach a minister's desk.
Of course, the ultimate passage of the legislation will not be without some degree of controversy, but the marginal cost of continuing at this stage will be insignificant compared to the furore over the Bill's introduction - a sunk cost at this stage. Failing to follow through would leave ministers - and indeed Prime Ministers - high and dry without the ability to exercise essential state functions within their own country. Such an outcome would be neither in the national interest, nor in the interests of Northern Ireland itself.
Only by learning the lessons of the past and anticipating the challenges of the future can a deal today be turned into a lasting good outcome for Northern Ireland tomorrow.