THERESA May’s Brexit ultimatum – my deal or no deal – has been denounced by the SNP as a “reckless false choice” with a range of voices highlighting the economic calamity of leaving the EU without an agreement.

Today at the Liberal Democrat conference Sir Vince Cable will accuse the hardline Brexiteers of indifference at the dire effects of withdrawal; “years of economic pain justified by the erotic spasm of leaving the European Union”.

His colleague, Sir Nick Clegg, bemoaned how the Tories had abandoned their One Nation approach and had become an English National Party, indifferent to Scotland and Wales.

The former Deputy Prime Minister told a conference fringe in Brighton that if the right-wingers continued to have the whip hand in the party, then separatism, which he had always opposed, would “flourish”.

“So, there’s a real danger we don’t lose our place in one Union, we lose our own Union as well,” he warned.

Appearing on a BBC Panorama programme last night to mark six months until Brexit Day, Mrs May expressed confidence MPs would back her Chequers Plan. On Thursday, she will discuss it with fellow EU leaders over dinner at an informal summit in Salzburg.

The PM said: "I believe we'll get a good deal; we'll bring that back from the EU negotiations, put that to Parliament. The alternative to that will be not having a deal because a) I don't think the negotiations will have that deal, and b) we're leaving on March 29 2019."

But Sir Nick accused her of presenting Britain with a "con trick" choice on Brexit between “my fudge or the abyss”. He insisted: “We always have more choices if only we are prepared to take them.”

Ian Blackford, the SNP’s Westminster leader, was equally aghast at Mrs May’s ultimatum.

“It would be utterly unforgivable to impose this reckless false choice. Theresa May must ditch her hard Brexit plans and get behind the SNP's compromise proposal to stay in the single market before it is too late," declared the Highland MP.

His Nationalist colleague, Michael Russell, the Scottish Government’s Constitutional Relations Secretary, will today warn that Mrs May is leading the UK into a “chaotic blind Brexit” with no clarity on the country’s future.

“It is misleading and the height of arrogance for the Prime Minister to say the only alternative to no-deal is the Tories’ flawed and unworkable Chequers Plan, which even they don’t agree on.

“The EU has been clear that the key economic planks of the Chequers Plan are not acceptable. That means there’s a growing danger that we are heading for a chaotic blind Brexit in which Scotland is dragged out of the EU with key questions on trade and the future relationship undecided and in the hands of hard-line anti-Europeans,” he added.

This morning, the Commons Brexit Committee issued a report urging the PM to consider alternatives to her Chequers blueprint, which kept Britain aligned to Brussels to avoid a "chaotic and damaging" no-deal Brexit if her plan were rejected. Three committee members – Tories Jacob Rees-Mogg and Andrea Jenkyns and the DUP’s Sammy Wilson refused to endorse it.

The report came after the International Monetary Fund said all likely Brexit scenarios would "entail costs for the UK economy" but that a disorderly departure could lead to "a significantly worse outcome".

Chancellor Philip Hammond also warned a no-deal would jeopardise the entire economic recovery since the 2008 financial crash.

Earlier, No 10 slapped down Boris Johnson after he again used his Daily Telegraph column to attack the Chequers Plan, saying under it Britain was "heading full throttle for the ditch with a total write-off of Brexit".

He complained the requirement for a "backstop" arrangement at the Irish border would leave the UK a "vassal state" of the EU.

But Downing St pointed out how the former Foreign Secretary had backed the plan while in Cabinet.

"At the time, he congratulated the Prime Minister for her determination in securing the deal. He remained in Government for a full seven months after the joint report was agreed,” said Mrs May’s spokesman.

Meanwhile, Labour MP David Lammy, his party colleague MEP Clare Moody and Tim Roache, the GMB General Secretary, together with party members, campaigners and activists will call for a People’s Vote at a march and rally in Liverpool on Sunday as the Labour Party conference kicks off.

Pressure is mounting for Jeremy Corbyn and the party leadership to change policy and formally back a second referendum.