The House of Commons will this week on Tuesday and Wednesday consider whether or not to accept the fundamental amendments made by the House of Lords to the EU withdrawal bill. Some have likened the significance of the debates to the Norway debate of May 1940, which displaced Neville Chamberlain replacing him with Winston Churchill.

The government will probably secure reversal of the most radical of the Lords amendments – committing Britain to retain the EU Charter of Fundamental Rights after Brexit and to remain in the European Economic Area with Norway, Iceland and Liechtenstein. It may be defeated on the amendment requiring it to “inform Parliament of the steps it has taken to negotiate British participation in a customs union”, but it can live with that, since the amendment does not commit the government actually to remain in the customs union.

But the most dangerous amendment, from the government’s point of view, is that proposed by Lord Hailsham, which would require both houses to approve the agreement by 30 November 2018, and to pass an act providing for the implementation of the agreement by 31 January 2019.

This seemingly innocuous amendment would give the House of Lords, around 80% of whose members support Remain, a veto over Brexit. The Lords have the power to refuse to consider a motion to approve the withdrawal agreement; they could delay passage of the bill providing for its implementation. The amendment would also allow parliament to refuse to accept any agreement. In consequence, Britain would remain a member of the European Union in defiance of the outcome of the referendum.

More fundamentally, the Hailsham amendment would substitute parliament for government in the negotiating process. If parliament refused to approve the agreement, it would require the government to return to the negotiating table to secure a better one. That is a constitutional absurdity. Parliament’s role is to scrutinise legislation and policy; 650 MPs, still less 800 peers, cannot themselves negotiate.

In international affairs, Britain must be represented by the government, not by parliament, which has never, in all our history, negotiated a treaty. Only those directly involved in the negotiation can judge what is attainable and what is not. They cannot make these judgments if they lack the authority to decide.

The Hailsham amendment would weaken the position of British negotiators, once it became known that any agreement concluded could be overturned by a parliament in which Remainers predominate.

The government has accepted that parliament, like the European Parliament, must have a meaningful vote on the withdrawal agreement. But just as the European Parliament does not seek to bind the commission, so also parliament must not seek to bind the hands of ministers. The government’s position is that if the withdrawal agreement is defeated by parliament, then instead of re-negotiation, Britain will exit from the European Union on 29 March 2019 without a deal.

But it is difficult to believe that the government could survive such a defeat. Indeed the Commons, in a hung parliament, could at any time, throw the government out, as it did the Callaghan government in March 1979. The consequence, were that to happen would, under the Fixed-term Parliaments Act, be either a weak minority Corbyn government or a general election.

The Hailsham amendment, therefore, lights a powder keg that threatens both Britain’s negotiating position with the EU and the survival of the government. But it does draw attention to the fundamental problem facing the government, which is seeking to implement an instruction from the British people in the Brexit referendum to which the majority of MPs and peers are opposed. For the first time in British constitutional history, parliament is passing legislation in which it does not believe.

But what has been decided by the people can be undone only by the people. It cannot be undone by parliament. All that parliament can do is to give the people a chance to decide on the deal.

Speaking in Berlin on Thursday, Sir Alan Duncan, the Foreign Office minister, said: “It would just be possible to ask the people in a referendum if they liked the exit deal or not.”

That is, indeed, the only way in which the conflict between parliament and government, and indeed the conflict within the Conservative party, can be resolved.

Vernon Bogdanor is professor of government at King’s College, London. His book on the constitutional consequences of Brexit will be published by Tauris in 2019