Theresa May has issued a brutal smackdown to Boris Johnson, declaring he has no new ideas on Brexit and is not a “serious” leadership contender.

In Number 10’s most pointed ever criticism of the former Foreign Secretary, the Prime Minister’s official spokesman let rip after Johnson savaged her Chequers compromise plan for the UK quitting the EU.

With remarks authorised by May herself, her spokesman said Johnson’s latest criticism in the Daily Telegraph had failed to come up with an alternative solution.

“Boris Johnson resigned over Chequers. There are no new ideas in this article to respond to,” he said.

“What we need at this time is serious leadership with a serious plan and that’s exactly what the country has with this Prime Minister and this Brexit plan.”

Asked if No 10 was saying Johnson not a serious politician, the spokesman replied: “I am simply putting forward that she’s a serious Prime Minister putting forward serious proposals.”

“The Chequers proposals are the only credible and negotiable plan which has been put forward and will deliver on the will of the British people.”

Johnson sparked the angry Downing Street backlash by declaring her Chequers plan – named after her country home where it was hammered out by the Cabinet – she had “gone into battle with the white flag fluttering over our leading tank”.

He wrote that the UK has “not even tried” to get the right deal on the Northern Ireland border issue, claiming the whole plan was a “fix” that would lead to “victory” for Brussels. 

Johnson quit his Government post days after he had agreed the Chequers plan. Tory Brexiteers have privately criticised him for waiting for Brexit Secretary David Davis to resign first.

No 10 this weekend underlined its suspicion that Johnson was plotting a Tory leadership challenge under the guise of a national campaign to block her proposals.

Former Johnson elections guru, Sir Lynton Crosby, is reported to have sent a key official to work with Brexiteer MPs in the backbench European Research Group (ERG), chaired by Jacob Rees-Mogg. 

May and Johnson clashed repeatedly when he was Mayor of London and she was Home Secretary but had put aside their differences once he pulled out of the Tory leadership race in 2016 and appointed him to the Cabinet.

Conservative MP Sarah Wollaston, who chairs the Commons health select committee, was scathing about Johnson’s latest remarks.

Home Secretary Sajid Javid gave May his loyal support on Monday and underlined No.10′s position.

Asked if Johnson’s column had helped or hindered a successful Brexit, Javid replied: “The thing that’s helpful is for us all to support the Prime Minister with her plan and make sure it’s getting a fair hearing.

“Those who think there’s a different way they need to properly set out what alternatives there might be.

“But right now this is the plan being put forward by the UK Government. It’s still be considered by all the different bits that make up the EU. And let’s see what they say.

“But that’s the plan. That’s the one everyone should be uniting behind.”

Former Home Secretary Amber Rudd also waded into the row, saying Johnson “has no solution” to the Northern Ireland border problem or the wider issue of Brexit.

“This isn’t a strategy. It’s not a plan. Once again it’s a case of leap before you look. There’s absolutely no proposal here.”

Rudd also became the first senior former Cabinet minister to back a plan by backbencher Nick Boles to keep the UK in the European Economic Area (EEA) in the short term, while it negotiates a long-term free trade deal.

Rudd said that Chequers was the best option but if it couldn’t get support then a useful alternative was that put forward by Boles and others: “I think they are making a good proposal there. It is preferable to no deal.”

Boles, a close ally of Michael Gove, told BBC Radio 4′s Today programme: “We can’t get to Nirvana in one step…What I want is a plan that’s workable.”

Asked by HuffPost if the PM had a view on the Boles plan, her spokesman said: “The British public voted to take back control of its borders and bring an end to free movement and membership of the EEA doesn’t allow for that.”

“The proposal we have put forward is one which we believe is negotiable and which can carry the support of the House of Commons.”