More than 60 Tories have warned the UK must have full control over laws after Brexit and must not become a “rule taker”.
Britain must not be stopped from negotiating and signing trade deals during any transition period, they said.
The letter was sent by the European Research Group (ERG), made up of Tory Eurosceptics, and was signed by 62 MPs, including former Cabinet ministers Priti Patel and Iain Duncan Smith.
It includes a number of “suggestions” for securing a successful Brexit, including “taking control” of World Trade Organisation tariff schedules that regulate trading.
The Government should publish a mandate showing how the UK wants to approach trade with the rest of the world.
“The UK must be free to start its own trade negotiations immediately,” the letter adds.
“The UK should negotiate as an equal partner. Ministers may not want or be able to accept the EU’s timing and mandates as fixed, and should be able to set out alternative terms including, for example, building an agreement based on our World Trade Organisation membership instead.
“Any ‘implementation period’ should be based on WTO principles. Any implementation period must not restrain the UK from negotiating or signing other trade agreements.”
The Prime Minister will hold a meeting of the Brexit sub-committee on Thursday to thrash out agreement on the next stages of the process.
Former Liberal Democrat leader Tim Farron, a supporter of the Best for Britain against Brexit, said: “It seems the Prime Minister has one arm tied behind her back by the Tory militants who are now nakedly acting like a party within a party.
“Not satisfied with hijacking the referendum result for their own extreme ends, now May’s militants, now called Maymentum, are hijacking the Conservative party – one nation pro business common sense Tories must fight back for the good of our country.
“And the PM must listen to the people, not to the far-right faction who are launching this shameless coup.”
It comes as a report by Brexit-backing economists said that quitting the European Union will boost the economy by up to 4%.
Economists for Free Trade said officials have failed to take account of the Government’s plans for a Brexit deal based on an enhanced version of the agreement with Canada when drawing up forecasts.
It said that instead of predictions the country could take a hit of up to 8%, Gross Domestic Product is projected to increase by at least 2% over the 15 years after Brexit once the plans are factored in.
Mr Duncan Smith said: “This new study by some of our most respected economists, who have been right in the past on key issues such as the euro and the immediate effects of Brexit, deserves to be taken very seriously.
“It suggests that we should all be highly sceptical of Project Fear Mark 2 – the Treasury-led operation by Whitehall officials to discredit Brexit and browbeat ministers into the softest of departures from the EU. It shows that far from depressing UK economic growth in the years to come, escaping from the EU will boost domestic economic growth and raise living standards right across the country, particularly for the poorest.”