Not for the first time it has been left to the unelected House of Lords to remind the Government that we are supposed to live in a democracy. A cross-party committee of peers has advised Theresa May that it would be “constitutionally inappropriate” for her to invoke Article 50, triggering the Brexit process without giving Parliament the right to a vote on the terms proposed. The Prime Minister has imperiously rejected this and made clear that she intends to move Article 50 in her own time, using royal prerogative. This is a royal abomination.

Royal prerogative is a constitutional anachronism (rather like the unelected Lords) which theoretically allows prime ministers to negotiate treaties and declare wars without consulting Parliament. But no one seriously believes that, in this day and age, prime ministers should continue to exercise these arbitrary rights, dating from the Divine Right of Kings. In our system, Parliament (the elected bit, that is) is supposed to be sovereign, not the Queen or any prime minister in place of the monarch. As the Lord’s constitutional committee says, it has become constitutional practice for important decisions on war and peace and international relations to be approved by Parliament, as was the case with Iraq and Syria.

The referendum on EU membership was anyway purely advisory, which doesn't mean that Parliament can reverse it. The decision of 17.5 million voters must be honoured. However, it is essential that Parliament should ratify it and that it should be free to scrutinise the terms under which Brexit is negotiated. This does not mean abolishing Brexit; it means putting it on a sound legal basis. It would seem self evident to most people that MPs should have a say in the terms under which Article 50 is invoked. The key issue of whether or not Britain should remain in the European single market (ESM) after Brexit is central and must be debated in parliament.

But Mrs May thinks differently. She will use the absolutist powers of a monarch, “the Crown in Parliament”, to push through Brexit on her terms without giving elected members a sniff. Not only will parliament not be allowed to vote on Article 50; MPs might not even be informed of the state of negotiations. The Brexit Secretary, David Davis, told MPs this would be impractical, and that negotiations have to be “confidential”. That is outrageous. Mr Davis has no right to supplant the elected Parliament.

Brexit has been less a referendum than a palace coup. Mrs May is using the plebiscite as a means of circumventing Parliament, which sets a disturbing precedent; not just Westminster but the Scottish Parliament too, which will also be denied a say on Article 50. All that talk about a “common UK approach” promised by Mrs May in her Bute House visit in June turns out to be so much hogwash. The “consultation” with Holyrood will be a one-way affair: I consult, you listen.

Yesterday the First Minister, Nicola Sturgeon, speaking to MSPs at Holyrood, repeated her insistence that Scotland needs to remain in the ESM. If not, she says: “The impact on the economy, jobs, trade, investment, universities and the lives of EU nationals living here and of UK citizens living in other EU countries will be severe.” She is right. Through all the confusion and obfuscation of the Brexit imbroglio, the one thing that looks crystal clear is that Britain will not be remaining in the ESM.

As EU President Jean-Claude Juncker made clear yesterday in his state-of-the-union address to the European Parliament, there can be no “a-la-carte” approach to the single market free trading zone. Countries that seek access to the ESM must observe the four freedoms of capital, goods, service and, above all, people. Free movement is non-negotiable. The EU cannot cave in on this central principle without it applying to the remaining 27 states. This would be chaotic, coming so soon after the refugee crisis that caused barbed-wire borders to be erected by some central European states. Caving into British Brexiters on free movement would fuel what Mr Juncker called “galloping populism” across the EU.

This is why Mr Davis, has said it is “highly improbable” that Britain will continue to have access to the single market. Nor will Britain be heading into the European Economic Area (EEA)/European Free Trade Association (Efta) grouping of countries like Norway that remain in the single market but are not members of the EU. Mr Davis told MPs that Efta was “not an option”. The reason is that, as a member of the EEA, Britain would have to be in the single market and be subject to free movement. Brexiters are in truth largely little Englanders who see halting immigration as the number-one priority; Brexit means keeping foreigners out.

Not everyone agrees with them, certainly not the Scottish people. And Mr Davis is usurping the UK Parliament's role. The EU referendum was not about the single market; it was about membership of the EU. It is not acceptable for Brexit ministers to elide the two. It is almost certainly the case that a majority of British voters would favour maintaining free trade with the EU, and the freedom of the 1.3 million British expatriates to live and work in the EU. In the absence of a question on this in the June referendum, it should be up to Parliament and Parliament alone to decide whether Britain should seek to remain in the single market.

This is a serious constitutional issue. It is no longer just about the EU, but about the nature of parliamentary democracy. One reason Mrs May hopes to sidestep parliamentary scrutiny may be that she realises any change in the law that affects Scotland would have to be ratified also by the Scottish Parliament in a legislative consent motion. This is the convention that any act of Westminster that cuts across the powers of the Scottish Parliament requires the consent of MSPs. In the case of Brexit, the consent of Holyrood will be hard won.

The Prime Minister probably thinks this is an obstacle she would rather avoid. Well, that's just too bad. This is a democracy, and it would be a dereliction of duty if the Scottish Parliament were to accept this lock-out. After all, Westminster remains sovereign under the Scotland Act and, if Holyrood withholds legislative consent, the Commons can always vote to overrule it. But it might be the case that MPs in Westminster will be reluctant to impose a diktat on Holyrood for fear that it would spark demands for a second Scottish independence referendum.

We are beginning to see why former Liberal Democrat leader Nick Clegg said that another Scottish independence referendum looks “compelling”. We are heading for tortuous constitutional waters in the course of which relations will be strained to breaking point between the Brexit dictatorship in Downing Street and the elected legislatures. Whether in Westminster or Holyrood, MPs must stand firm. Say no to no say.