Boris Johnson has resigned from his position as British Foreign Secretary in a fresh blow for Prime Minister Theresa May's Government.

Johnson followed Brexit Minister David Davis in resigning over the PM's masterplan for Britain's future outside the European Union.

Davis and his deputy quit just two days after May announced she had finally united her quarrelsome Government behind a plan for a divorce deal with the EU.

Johnson gave a scathing verdict on Theresa May's Brexit plans tonight as his resignation letter said the "Brexit dream is dying, suffocated by needless self doubt".

The ex Foreign Secretary said May was putting Britain on a track to having "the status of a colony".

Johnson said Britain was destined for a "semi Brexit" if the blueprint agreed by the Cabinet at Chequers on Friday is delivered.

He said selling the policy would "stick in the throat" and he could not stand by waving "white flags" of surrender, meaning he had to quit.

The damning criticism in his resignation letter was Johnson's first explanation for his stunning decision to walk out of Government this afternoon.

He was under fire as even David Davis said he did not need to quit as Foreign Secretary - prompting claims he resigned to further his own ambition.

Johnson rocked the Government by becoming the second Cabinet minister to resign in less than 24 hours.

The resignations are a stunning reversal from Friday's special Cabinet at Chequers when all ministers signed up and Johnson proposed a toast to the deal - even suggested a joint newspaper column with Chancellor Philip Hammond.

Instead May was left scrambling to shore up her ailing Premiership amid fierce rumours she is set to face a no confidence vote, possibly within hours.

After two hours in the Commons defending her plans, the PM faced Tory MPs behind closed doors - with many warning her offer gave too much away.

In an apparent boost for the embattled PM, chairman of the powerful 1922 Committee, Sir Graham Brady, is said to have confirmed at the meeting currently he has not received the 48 letters from MPs that would trigger a no-confidence vote.

Resignation letter

In his resignation letter, Johnson said: "Brexit should be about opportunity and hope. It should be a chance to do things differently, to be more nimble and dynamic, and to maximise the particular advantages of the UK as an open, outward-looking global economy.

"That dream is dying, suffocated by needless self-doubt."

Johnson said crucial decisions on preparing for a no deal Brexit had been "postponed" - meaning Britain was headed for a "semi-Brexit".

He said: "We are now in the ludicrous position of asserting we must accept huge amounts of precisely such EU law, without changing an iota, because it is essential for our economic health - and when we no longer have any ability to influence these laws as they are made.

"In that respect we are truly headed for the status of colony - and many will struggle to the economic or political advantages of that particular arrangement."

Johnson said he accepted his side of the argument lacked support to win out at Cabinet on Friday.

He told the Prime Minister: "As I said then, the Government now has a song to sing.

"The trouble is that I have practised the words over the weekend and find that they stick in the throat."

There is speculation that either Trade Secretary Liam Fox or Environment Secretary Michael Gove could be shifted to take Mr Johnson's role.

How could Theresa May be ousted as Tory leader?

Theresa May faces a mortal threat to her leadership of the Conservative Party and Government today.

A Tory leadership contest can be called in one of two ways - if May resigns or if MPs force and win a vote of no confidence in her.

Calling votes of no confidence is the responsibility of the chairman of the 1922 Committee, which includes all backbench Tory MPs.

Chairman Graham Brady is obliged to call a vote if 15 per cent of Tory MPs write to him calling for one - currently 48 MPs.

The process is secret and only Brady knows how many letters he has received.

The procedure was last used in 2003 when Iain Duncan Smith was ousted as Tory leader.

If May is ousted, any MP is eligible to stand.

Conservative MPs will then hold a series of ballots to whittle the list of contenders down to two, with the last place candidate dropping out in each round.

The final two candidates are then offered to the Tory membership at large for an election.