Boris Johnson is a prime minister without a mandate. He has never faced an election and has lost every vote he has put to the House of Commons. Yet time and again he has proved his willingness to ride roughshod over parliament in order to get his way.

Yesterday was no different: MPs were disgracefully given just a few hours to scrutinise the terms of the most important decision the country has faced in decades. But he miscalculated badly. Parliament reasserted its sovereignty, voting to withhold approval of his EU deal until MPs have a chance to scrutinise the relevant legislation, effectively forcing him by law to request an extension from the EU.

Johnson would have us believe that he achieved the impossible in Brussels last week in agreeing a new deal with the EU. But this was no Houdini feat: all he negotiated was a deal that was always possible from the perspective of the EU. In agreeing that Northern Ireland will effectively be part of the EU's customs union and the single market in goods, Johnson has trashed his pledge that he would avoid a border between Northern Ireland and the rest of the UK at any cost.

He has used underhand strategy after underhand strategy to evade scrutiny. In September, he tried to shut down parliament for five weeks, an action ruled unlawful by the courts. He left it until the last possible moment to make the concession on Northern Ireland that resulted in a new deal. He clearly believed that his best chance of getting this substandard deal through parliament was to strongarm MPs into voting for it, using the threat of an imminent no-deal cliff edge.

Reading between the lines of the deal, it is clear that Johnson's intended final destination is a hard Brexit with a minimal free trade agreement. Why else solve the Irish border issue by introducing a border in the Irish Sea, rather than keeping the whole UK in the customs union?

In doing so, his deal puts the union at risk. The Stormont assembly has not sat since power-sharing broke down in 2016; by undermining the governing principle of cross-community consent, the deal could further upset the fragile political balance in Northern Ireland and make Irish reunification more likely. It will also give further succour to the Scottish independence movement, whose support is growing among Scots who voted to remain in 2016. If provisions can be made for Northern Ireland to remain in the single market and customs union, why not for Scotland, which also voted to stay in the EU?

The government has refused to publish an evaluation of the deal's economic impact. But independent forecasts estimate that it would come at a huge cost in growth, jobs and wages. The government's own estimates show that new free trade deals with other countries cannot compensate for more than a fraction of the economic cost of leaving the single market and customs union. Free trade agreements generally cover only goods, yet services make up 80% of the UK economy. And signing an agreement with countries such as the US would likely force the UK into becoming a rule-taker from economies far more deregulated than our own

Nor does this deal "get Brexit done"; it will take years to negotiate a free trade agreement with the EU, not least because every member state will have a veto. Yet Johnson has refused to commit to asking for an extension to the transition should there be no agreement by December 2020. Former chancellor Philip Hammond was right to warn of "a heavily camouflaged no deal at the end of 2020".

Johnson is evidently happy to sacrifice two pillars of British conservatism - the preservation of the union and responsible management of the economy - on the altar of Brexit in order to maximise his own chances of political survival. But MPs have bought themselves time to scrutinise his deal properly. It does not represent some "will of the people". The potential breakup of the UK; a poorer and more unequal nation: this is not what voters were promised back in 2016. For Johnson's Brexit to have any shred of legitimacy, it must be put to voters in a confirmatory referendum.

So far, it has been the actions of backbench MPs such as Letwin and Hilary Benn that have stopped Johnson's authoritarianism in its tracks. In squabbling about the timing of any vote of no confidence, and who might take over as a temporary prime minister, opposition leaders have hardly covered themselves in glory. But at the last hour, parliament has created a window in which Labour and the other opposition parties can act. The only way in which Johnson's deal could be legitimised is in a referendum. MPs must deny him parliamentary approval for his deal unless he agrees to put it to voters.