Civil servants and politicians who are dealing “directly” with Brexit are “horrified” at the potential for a “disastrous” outcome to negotiations with the European Union, Sir Vince Cable has claimed.

The Liberal Democrat leader said they realise the UK does not have a strong hand in the talks and are “worried about the future of the country”.

The former Business Secretary spoke as thousands of demonstrators joined the anti-Brexit People’s March for Europe through central London, which ended with a rally in Parliament Square.

Before addressing Lib Dem activists who were joining the march, Sir Vince said: “I think a lot of the people who are dealing with it directly and who don’t necessarily have a political axe to grind are pretty horrified actually because they see the potential for disastrous negotiations, they realise Britain doesn’t have a very strong hand in these negotiations and they are worried about the future of the country, as I am, as these people are.”

Asked who exactly he was talking about, Sir Vince replied: “I’m talking about people in the other political parties, they go along with Brexit because they say, well people in my constituency voted for it, but deep down they are really, really worried about the consequences, and of course people who are not political.”

Sir Vince’s comments came at a critical moment for Brexit, with MPs set to vote on the Repeal Bill, formally known as the EU (Withdrawal) Bill, on Monday for the first time, and amid concerns over the progress of negotiations.

Foreign Secretary Boris Johnson on Friday made clear the Government’s urgent wish to negotiate trade now, suggesting Article 50 sets out a legal duty to discuss the future relationship concurrently with withdrawal issues including citizens’ rights, a financial settlement and the Irish border.

But he spoke after senior European figures, including EU chief negotiator Michel Barnier, voiced scepticism that talks would move on to trade relations by October, as hoped, as they have so far failed to deliver “decisive” progress on withdrawal issues.

Meanwhile, Theresa May was forced to rule out sacking ministers or aides suggested to be involved with a letter signed by between 30 and 40 Tory MPs which set out demands for what was seen as a “hard” Brexit that differed with Government policy.

Sir Vince said there is now a “swelling” of people who are “deeply alarmed” about where negotiations are heading and the “confusion and disunity” in the Government.

Public opinion will shift as more businesses hold back investment and “high quality” EU citizens join an “exodus” out of the country, he said.

“We’ve already seen the big devaluation [of the pound], and that had has quite a significant impact on people’s living standards,” Sir Vince said.

“There’s already plenty of evidence that companies are not investing at the moment. They are holding back because of the uncertainty, and those that depend on the single market, the customs union obviously are going to hold back, that’s very understandable.

“You are beginning to see more and more evidence of an exodus of high-quality, highly trained people from the European Union because of uncertainty about the future, all of these things are happening now and this is just the beginning.”

Brexit: the deciders Show all 8 1 /8 Brexit: the deciders Brexit: the deciders European Union's chief Brexit negotiator, Michel Barnier Getty Brexit: the deciders French President Emmanuel Macron Getty Brexit: the deciders German Chancellor Angela Merkel Reuters Brexit: the deciders Commission President Jean-Claude Juncker EPA Brexit: the deciders The European Parliament's chief Brexit negotiator Guy Verhofstadt Getty Brexit: the deciders Britain's Prime Minister Theresa May Getty Images Brexit: the deciders Britain's Chancellor of the Exchequer, Philip Hammond PA Brexit: the deciders After the first and second appointed Brexit secretaries resigned (David Davis and Dominic Raab respectively), Stephen Barclay is currently heading up the position PA

Meanwhile, more than 30 Labour MPs challenged David Davis over his “downright absurd” claim after last year’s referendum that Britain could negotiate a free trade area “massively larger” than the EU, to come into force before the end of negotiations.

Mr Davis made the claim last July before being appointed Brexit Secretary.

Since then the Government and potential trading partners have been forced to admit no deals can be negotiated while the UK remains a member of the EU.

Ministers have also said Britain will not be able to implement any agreements until after a proposed customs transition period, which will follow the formal exit in March 2019.

In a letter to Mr Davis, the MPs, who support the Open Britain campaign for close ties with the EU, said: “The Government’s Brexit strategy has for months been characterised by delusional wishful thinking.

“But probably no pledge you have made is as downright absurd as the promise that Britain would be in the process of concluding trade deals with major economies over a year before we even leave the EU.”

One of them, Chuka Umunna, added: “The claims and promises the likes of David Davis made to the British people are simply not deliverable.”