Boris Johnson could be Prime Minister in as little as nine weeks after leading Tories call for Cameron to go now

Boris Johnson could become Prime Minister in just nine weeks’ time after Conservative MPs called for David Cameron to be gone and a new leader in place by September.

As MPs returned to Westminster for the first time since the historic Brexit vote, the party’s influential 1922 committee said a new Conservative leader should be in place by 2nd September, with nominations set to open on Wednesday.

Announcing his resignation last week, Mr Cameron had said a new Prime Minister should be appointed to negotiate the terms of Britain’s exit from the EU by October. But the shorter timescale, which must be approved by the party’s board on Tuesday will favour Mr Johnson, the frontrunner in the race, allowing him to harness the momentum from Leave’s referendum victory to garner support among the 125,000 Conservative members who will elect the next leader, and the next PM.

Addressing MPs in the House of Commons, Mr Cameron said there could be “no doubt about the result” of the referendum but warned of difficult days ahead.

A special Whitehall ‘Brexit unit’ will be set up to explore the options facing Britain in its new relationship with the EU, he said, admitting that it would be “the most complex and most important task the British civil service has undertaken in decades”. However all key decisions will wait until the appointment of a new Prime Minister, Mr Cameron said.

Boris Johnson, who was not in the House of Commons chamber, is considered the frontrunner to replace Mr Cameron and is expected to announce his candidacy this week.

The new leader will come under immediate pressure to call a general election. The former Liberal Democrat Deputy Prime Minister, Nick Clegg, said it would be wrong that only members of the Conservative party should be able to appoint the new Prime Minister “of a new government, with new priorities” and called for an early general election this year.

Mr Cameron, who will remain as an MP after standing down, will remain neutral during the Conservative leadership contest, Downing Street confirmed. He told MPs it would be for the next Prime Minister to determine whether a general election is called this year.

It will also be up to the new Prime Minister to activate Article 50 – the formal procedure by which member states can leave the EU – and to negotiate the terms of Britain’s new relationship with the bloc.

The key battle-line will centre on Britain’s access to the European single market. Mr Cameron told MPs that it would be “one of the single most important decisions” for the government, emphasising the single market’s importance for the economy and jobs market.

Mr Johnson has claimed the UK could still have access to the single market – but to do so would entail adopting a relationship with the EU similar to Norway’s, and still being bound by freedom of movement rules.

After the Leave campaign led by Mr Johnson fought the referendum campaign on a promise to cut immigration, staying in the single market at this cost would be seen as a betrayal of Brexit voters. Andrea Leadsom, the Conservative Brexit campaigner, who has said she is considering a leadership bid, has said blocking freedom of movement should be a red line in the EU negotiations.

“The decision we’re going to have to take, and it will be for the next government, about how we get the best possible access to the single market, I think will be one of the single most important decisions that the government must take on,” Mr Cameron said. “Because we must bear in mind the importance of safeguarding our economy, its trade links and its jobs.”

German Chancellor Angela Merkel, French President Francois Hollande and Italian Prime Minister Matteo Renzi insisted there could be no formal or informal talks with Britain’s new relationship until the Article 50 had been activated. Mr Cameron will meet fellow EU leaders for talks in Brussels tomorrow.

Mr Cameron also used his Commons statement to condemn the rise in anti-migrant incidents reported since the Brexit vote, including an incident of racist graffiti daubed on a Polish community centre.

A Downing Street spokesperson said that many MPs had received reports from their constituencies and communities of migrants being intimidated or “told they need to go home”.

“We are absolutely clear and we need to reassure communities across Britain…this government will not tolerate intolerance,” the spokesperson said.

Saying that Britain was a tolerant, inclusive nation, the spokesperson added: “Those views and that value of our nation existed long before we were members of the EU and we should hold fast to that value and stand up for that value in the days and weeks ahead.”

Downing Street has also ruled out a second EU referendum, saying it was “not remotely on the cards”, and said that a second Scottish independence referendum was “the last thing Scotland needs”, after the Scottish government insisted they would not allow the country to be taken out of the EU.