Boris Johnson faces heavy criticism ahead of his major Brexit speech as one Commons committee chair asks why he is being taken seriously following the infamous pre-referendum assertion that Britain sends £350m a week to the EU.

The comments come as the Foreign Secretary prepares to deliver a speech on what his allies claim is a liberal vision for Brexit and carries the warning that reversing the referendum result of 2016 will be a “disastrous mistake".

Yvette Cooper, the senior Labour MP and chair of the Home Affairs Committee in Westminster, said: “The problem with that is, from the point of view of a committee chair, we've got this speech being made which doesn't seem to set out any detail.

“The Government cannot keep kicking the can down the road, we've got to actually have some practical details on it."

Referring to his pre-referendum claim – one that has been discredited by the chair of UK Statistics Authority – Ms Cooper told the BBC’s Radio 4 Today programme: “And to be honest given that everything he said about the bus I don’t know really see why we’re taking him seriously.”

In an attempt to reach out to pro-Europeans in Britain, Mr Johnson, a prominent Leave campaigner, will also add in his speech: “It is not good enough to say to Remainers – you lost, get over it; because we must accept that many are actuated by entirely noble sentiments.”

Extracts of Mr Johnson’s speech were also met with a lukewarm response from Thorbjörn Sohlström, the Swedish Ambassador to the UK, who warned that the Government’s Brexit red lines were “not so easy to marry with friction-free trade”.

He continued: “It's clear that if Britain will indeed leave both the customs union and the single market and perhaps take some distance from EU rules and regulations, there will be, I think, a degree of friction in the trade.

“There are strong incentives on each side to find a good arrangement and we will try to be as constructive as possible to achieve that sort of outcome, but I don't think it will be that easy.”

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And Green leader Caroline Lucas, a support of the Best for Britain organisation, added: “Boris is deluded if he thinks, he of all people can 'reach out to people who voted remain'. He lied to the public to further his own political goals - and people up and down the country are now picking up the pieces.

“It may be Valentine's Day tomorrow - but the British people have well and truly fallen out of love with Boris Johnson, and have had enough of his self-aggrandising nonsense.”

The Conservative MEP Daniel Hannan, a founding member of Vote Leave, however, said Mr Johnson was aiming his speech at those who felt “alienated and angry” about the referendum result.

What could the sticking points be in the Brexit trade deal?

Mr Hannan said: “He knows that a number of people feel kind of alienated and angry about the result and that's not something that any Leaver should feel good about.

“We want to try and carry as many people with us, it was a narrow outcome, it was a 48/52 vote that means we should try and find a consensus that both sides can at least live with.”