Theresa May has come under renewed pressure over Brexit on the eve of the Tory Party conference.

Foreign Secretary Boris Johnson has insisted that any post-withdrawal transition period must not last “a second more” than two years, while some senior Conservatives have said Britain should walk away from Brexit negotiations by Christmas if no serious progress is being made.

Mr Johnson said the UK should not have to abide by any new EU rules during a post-withdrawal transition period, and that Britain should not make payments to Brussels after it.

(Victoria Jones/PA)(Victoria Jones/PA)

He said there can be “no monkeying around” about withdrawal from the EU.

Mr Johnson, who insisted his stance was not a leadership pitch after a poll of activists showed him well favoured for the top job, said Brexit needed to happen quickly.

He told the Sun: “Am I impatient about it, do I want to get it done as fast as possible? Yes, absolutely. Do I want the delay to go on longer than two years? Not a second more.”

The move came as a letter to the Prime Minister from senior pro-Brexit Tory MPs and business figures in the Leave Means Leave group stated: “If the EU is not seriously negotiating a free trade deal by Christmas 2017, the Government should give formal notice that we will move to World Trade Organisation rules in March 2019.”

Scottish Conservative party leader Ruth Davidson. (Jane Barlow/PA)Scottish Conservative party leader Ruth Davidson. (Jane Barlow/PA)

Signatories calling for the hardline stance include former Brexit minister David Jones, and the issue is set to feature heavily at the Tory conference.

However, Scottish Tory leader Ruth Davidson called for “serious people” to take charge of Brexit process.

Labour leader Jeremy Corbyn said the Foreign Secretary’s stance threatened a trade war with the EU.

Theresa May(Joe Giddens/PA)

He said: “It seems a pretty dysfunctional Government. I can’t imagine what it’s like sitting around a table with their Brexit negotiating team because there are three or four people with three or four completely different opinions.

“Boris this morning seems to be saying two years maximum on the transition period and then no shadowing of EU rules. Well, that sounds to me like a threat to have a trade policy that undermines Europe.

“Therein lies the basis of a trade war of the future, therein lies a threat to thousands and thousands of jobs in Britain.

“You have got to have a serious, adult, grown-up relationship with Europe.”

Theresa May v Jeremy Corbyn: approval rating(PA graphic)

Liberal Democrat leader Sir Vince Cable said Mr Johnson’s intervention had undermined the Government’s Brexit negotiating position.

“Boris Johnson’s latest red line sends an appalling signal to EU negotiators who thought they were dealing with David Davis and Theresa May but now realise that the strings are being pulled by others,” he said.

“Senior Conservatives are displaying an abject failure to act together in the national interest and seem more motivated by selfish, personal ambition.”

The Foreign Secretary’s comments came as Mrs May is attempting to assert her grip on her party ahead of its annual gathering.

As the party faithful converge on Manchester, the Prime Minister insisted Labour was “unfit to govern” as she pledged to listen to the concerns of young voters after the Tories’ “disappointing” general election showing.

Mr Johnson is also urging workers to be given a pay hike as he insisted the current minimum wage of £7.50 an hour – rising to £9 by 2020 – is “not enough”.