David Cameron has come under increasing pressure from Nicola Sturgeon and Boris Johnson over his plans to keep the UK in the European Union.

The London mayor became the most high-profile Tory critic of the proposed new deal on Europe, saying that the Prime Minister was making the “making the best out of a bad job".

Another Tory MP warned he had just two week to salvage his reputation, amid "thin gruel".

In a sign of further trouble ahead the former defence secretary Liam Fox also predicted that as many as five cabinet ministers could oppose the Tory leader him and campaign for a ‘Brexit’.

Meanwhile, Ms Sturgeon and the leaders of the other devolved nations wrote to Mr Cameron urging him not to call a June referendum.

They argue that the campaign would clash with elections happening across the UK at the start of May, including to Holyrood.

Mr Cameron appealed to Tory MPs, saying that he would push for further EU reforms even if he secured an 'In' vote.

He also claimed that curbs on benefits for EU citizens could be implemented "shortly after" the referendum.

Sources have suggested that even fast-tracked the changes could take six months.

Under pressure from his own MPs, Mr Cameron insisted that his proposals would be legally-binding.

But he warned them that there was still “hard work, determination and patience” required to get the other 27 EU states to sign up to the deal.

Labour leader Jeremy Corbyn accused the Prime Minister of organising a “Tory Party drama that's being played out in front of us”.

"For all the sound and fury you have ended up exactly where you knew you would be - making the case to remain in Europe, which is what you always intended despite the renegotiating spectacle choreographed for TV cameras over the whole continent," he said.

But unlike the First Ministers the Labour leader urged Mr Cameron to name the date as June 23.

This is understood to be Downing Street’s preferred polling day.

No 10 is keen to hold the referendum before the start of Scottish school holidays.

Ministers are understood to be nervous that a delay could increase the chances of an ‘Out’ vote, especially with the growing migrant crisis affecting Europe.

Mr Cameron did secure support from Tory grandee and proEU MP Ken Clarke.

But Conservative MP Jacob Rees-Mogg said Mr Cameron had two weeks to save his reputation after serving up “thin gruel".

And Tory former cabinet minister Peter Lilley warned that the flagship ‘emergency brake’ on benefits risked being struck down by the European Court of Justice because it would conflict with freedom of movement.