Theresa May has vowed that her plans for Brexit will not be “derailed” as senior ministers prepared to formally consider the future shape of the UK’s relationship with the European Union.

The Prime Minister claimed her Government is “proving the doubters wrong” after EU leaders agreed to move on to the next phase of the Brexit negotiations.

Ms May said talks would now begin on an “implementation period” immediately after the formal date of Brexit – but backbench Tory Eurosceptics have already issued warnings that they will not accept arrangements which closely resemble continued EU membership during the transition to a new relationship.

Boris Johnson called for the Prime Minister to strike a deal with Brussels that would allow the UK to ditch EU laws, warning that being unable to diverge from the bloc’s regulations would leave the UK a “vassal state”.

Writing in the Sunday Telegraph, Ms May hit out at anti-Brexit campaigners who “want to talk Britain down”.

She said: “Amid all the noise, we are getting on with the job.

“In the face of those who want to talk Britain down, we are securing the best and most ambitious Brexit deal for our whole United Kingdom.

“And my message today is very clear: we will not be derailed from this fundamental duty to deliver the democratic will of the British people.”

The Cabinet will thrash outs its stance on a post-Brexit trade deal over the coming days, with Ms May under pressure from Brussels to provide clarity on the UK’s desired “end state” for the relationship it wants with the EU.

The Brexit “war cabinet” – a sub-committee of senior ministers chaired by Mrs May – will meet on Monday, with a meeting of the full Cabinet scheduled for Tuesday.

Ms May said: “Brexit allows us to seize the exciting opportunities outside the EU – with Britain in control of our borders, and setting our own laws – while building the new European economic and security relationship that I have proposed.

“So we will approach these discussions with ambition and creativity.”

Mr Johnson used a Sunday Times interview to set out his vision for a UK-EU trade deal that would ”maximise the benefits of Brexit“ by allowing Britain the freedom to diverge from Brussels’ laws.

He called for a deal that “gives us that important freedom to decide our own regulatory framework, our own laws and do things in a distinctive way”.

The Prime Minister said she made agreeing an implementation period a priority to allow businesses and families the time they need to adapt to a new relationship with the EU.

“I very much welcome the desire of the European Union to agree the precise terms of this period as soon as possible,” she said.

The EU’s negotiating guidelines make clear that the bloc expects the UK to observe all of its rules – including on freedom of movement – and accept the jurisdiction of the European Court of Justice (ECJ) during this time.

It also set up a potential clash with London over the Prime Minister’s hopes of negotiating early trade agreements with countries outside the EU, stating firmly that the UK will stay in the single market and customs union during transition and will ”continue to comply with EU trade policy“, which bars deals by individual states.

Chancellor Philip Hammond said a transition deal would ”replicate the status quo“ and that, although ”technically“ the UK would not be in the customs union or single market, it would effectively keep the same rules during an implementation period until the terms of a new deal can be put in place.

Brexit bonhomie as May finally seals agreement with EU leaders Show all 10 1 /10 Brexit bonhomie as May finally seals agreement with EU leaders Brexit bonhomie as May finally seals agreement with EU leaders Britain's Prime Minister Theresa May is welcomed by European Commission President Jean-Claude Juncker Reuters Brexit bonhomie as May finally seals agreement with EU leaders EU President Jean-Claude Juncker greeting Theresa May at the EU Commission in Brussels PA Brexit bonhomie as May finally seals agreement with EU leaders Britain's Prime Minister Theresa May poses for a picture with European Council President Donald Tusk REUTERS Brexit bonhomie as May finally seals agreement with EU leaders EU Commission President Jean-Claude Juncker walks behind British Prime Minister Theresa May EPA Brexit bonhomie as May finally seals agreement with EU leaders British Prime Minister Theresa May (L) and European Commission President Jean-Claude Juncker address a press conference at the European Commission in Brussels AFP/Getty Images Brexit bonhomie as May finally seals agreement with EU leaders British Prime Minister Theresa May speaks at a press conference with EU Commission President Jean-Claude Juncker EPA Brexit bonhomie as May finally seals agreement with EU leaders Britain's Secretary of State for Exiting the European Union, David Davis (L) and Michel Barnier (2-L), the European Chief Negotiator of the Task Force for the Preparation and Conduct of the Negotiations with the United Kingdom under Article 50 enter the room by the emergency exit to attend British Prime Minister Theresa May press briefing on Brexit Negotiations in Brussels. EPA Brexit bonhomie as May finally seals agreement with EU leaders Britain's Secretary of State for Exiting the European Union David Davis (L), Britain's Prime Minister Theresa May (2-L), European Commission President Jean-Claude Juncker (2-R) and European Union's chief Brexit negotiator Michel Barnier (R) in a meeting on Friday morning AFP/Getty Images Brexit bonhomie as May finally seals agreement with EU leaders European Union chief Brexit negotiator Michel Barnier speaks during a media conference at EU headquarters Brexit bonhomie as May finally seals agreement with EU leaders European Council President Donald Tusk addresses a media conference at the Europa building in Brussels AP

But he faced a backlash from prominent Brexiteers for his comments – even though the Prime Minister has already suggested in her Florence speech that the ”framework“ for the implementation period ”would be the existing structure of EU rules and regulations“.

Former Brexit minister David Jones condemned the Chancellor, claiming he ”appears only too ready to do Brussels’ bidding“ by signalling acceptance of the EU’s position.

Ex-Tory leader Iain Duncan Smith accused Mr Hammond of ”undermining“ Ms May, while prominent backbencher Jacob Rees-Mogg said the EU’s position would reduce the UK to the status of a ”colony“.

Ms May has also been warned she will face a string of defeats in the House of Lords over Brexit unless she adopts a more conciliatory approach to peers as they consider legislation on the UK’s withdrawal.

Former pensions minister Baroness Altmann and fellow Tory Baroness Wheatcroft said there had been “appalling insults” aimed at Brexit rebels in the Commons and warned: ”The Lords is unlikely to be receptive to bullying over a restricted timetable or vigorous whipping to toe the party line.“