Ever since she took office, Theresa May has sought to evade and avoid parliamentary sovereignty. Her assertion of “government sovereignty” has reached crisis point, and it is imperative it is rejected by both the Lords and Commons in this week’s debates on Brexit. The double-crossing of Dominic Grieve brings the issue to a head.

Last Tuesday, by his and other accounts, May promised Grieve there would be a legal commitment to a “meaningful” parliamentary vote in the event of a “no deal” Brexit. On this basis he was persuaded to withdraw his amendment to the EU withdrawal bill, which the government feared it would lose. On Thursday Grieve secured an acceptable legal text from ministers, which was suddenly withdrawn and replaced by a provision that in case of “no deal” the House of Commons would be allowed only to debate a “take note” motion, which would be unamendable and meaningless if the government chose to ignore it.

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This assertion of government sovereignty follows a now established pattern. May’s first step on the Brexit road was her implacable refusal to submit the EU withdrawal notice for parliamentary approval, claiming the royal prerogative. It took a path-breaking decision by the supreme court to force her into parliament. Worried that she might not have a majority to carry hard Brexit through the Commons, the prime minister then called an election, expecting a landslide victory. Instead she got a hung parliament, and has no credible mandate for anything the Commons does not authorise.

The reason May is now seeking to solidify “government sovereignty” in advance of the key Brexit votes in parliament this autumn is precisely because she is so weak in parliament. That is all the more reason not to grant her powers that no head of government has assumed since Charles I and James II in the 17th century.

If May’s proposal is carried, the House of Commons would be in a weaker position than if there were no advance legal provision whatsoever for what to do in the event of “no deal”. By constitutional practice, the Commons would be free to vote on whatever resolutions it thinks fit under normal circumstances, whereas May is seeking the Commons’ agreement to deprive itself of any meaningful vote in advance. To try to make an argument, ministers are objecting to the word “instruction” in the Grieve amendment. David Davis, once a poster boy for civil liberties, now asserts that it is wrong for the Commons to seek to “instruct” the government, and this might give rise to the unacceptable situation of MPs taking over the Brexit negotiations from ministers.

Ever the conservative, Grieve is proposing only that parliament should give instructions in the event that the government fails to negotiate any deal at all. As he rightly says, this would be a national emergency, and since – unless May’s new doctrine is accepted – parliament is sovereign, it has a duty as much as a right to say what should then happen. It would then be for the government to “execute” the will of parliament – which is why it is called the executive. It is absolutely legal and appropriate for the House of Commons to give instructions to the government in such circumstances. There are countless cases where it has done so over the centuries, and it is the essence of our parliamentary democracy.

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Davis himself has form on this issue. In 2012, in the argument over Britain’s EU budget contribution, he supported a successful backbench amendment calling on the government to “strengthen its stance” to reduce it. That’s an instruction. More explicit still are recent Commons’ resolutions on military action, an area where until the Iraq war MPs had often deferred to ministers. To assuage discontent among MPs, Tony Blair went to war in 2003 only after the Commons explicitly resolved that it “supports the decision of HM Government that the UK should use all means necessary to ensure the disarmament of Iraq”. “It is right that the House debate this issue and pass judgment,” Blair said.

The next time the Commons was asked to authorise military action – by David Cameron, against Syria in 2013 – it refused to do so, rejecting an explicit motion to “agree to military action”. When the motion was lost, Cameron immediately told MPs: “I believe in respecting the will of this House of Commons. It is very clear that the House does not want to see British military action. I get that, and the government will act accordingly.”

Tellingly, when May undertook limited military strikes against Assad’s regime earlier this year, she refused to summon parliament or table a resolution. Those opposed to such bypassing of parliament didn’t press the issue because the airstrikes had already happened by the time parliament met to debate them, and May indicated there would be no more. In retrospect, MPs would have been wise to register their strong objections, whether for or against the action, since it only encouraged May to approach Brexit in the same high-handed manner.

The irony is that Brexit is supposed to be about parliament “taking back control”. That fiction is being steadily exposed for what is in reality a power grab by the right wing of the Conservative party, which is seeking to suppress the party’s moderate and pragmatic MPs – who, May and Davis know, would be able to command a majority in the Commons if matters went to a vote.

That is why it is crucial that both houses of parliament support the Grieve amendment this week. We should do so in the tradition of the Bill of Rights 1689, which declared resistance to James II’s attempt to subvert the “laws and liberties” of this kingdom by assuming a power of suspending of laws without consent of parliament.

• Andrew Adonis is a Labour peer and former transport secretary