EU sources say a fresh plan for the bloc's future being put forward tomorrow by French President Emmanuel Macron could lead to the UK being offered a flexible new membership deal.
His blueprint is expected to win the backing of Angela Merkel, who is on course to return as German Chancellor following today's general election.
Tory Eurosceptics are concerned an EU attempts to scupper a full exit from the bloc are accelerating following Theresa May's decision to delay a full break with Brussels for at least another two years.
A senior Conservative MP said last night: "A serious effort to stop Brexit is now underway. I am very worried that we will now never leave the EU."
The drive for integration among core EU member states is expected to accelerate drastically now the German elections are out of the way.
President Macron is due to give a speech tomorrow at the Sorbonne University in Paris detailing his new vision for the EU's future.
He will say that France and Germany must press ahead with creating a core political and economic union at the heart of the EU.
The French president is expected to put forward specific proposals in about 10 policy areas including strengthening the economic and monetary union, deepening European defence, re-enforcing EU migration policy and increasing social and fiscal convergence in the bloc.
According to a senior British official in the European Commission, the French President will also revive the idea of an outer right for the bloc with more flexible rules for countries less enthusiastic about the rapid integration among core members.
“A two-speed Europe is his answer to the Brexit problem," said the commission official.
Mr Macron’s intervention follows the Prime Minister’s crunch Brexit speech on Brussels, announcing the Government’s offer of a two-year “implementation period” after Brexit during which Britain will remain subject to most EU rules and regulations.
Tory Eurosceptics are concerned that the transition will effectively delay Britain’s departure from the bloc and give opponents of Brexit more time to try to minimise the extent of the break with Brussels.
Now some Tories fear the Macron plan could mean Britain being offered a form of associate EU membership that includes some flexibility to introduce curbs on free movement for EU citizens to come to the UK.
Speculation about a possible-last ditch offer from EU leaders has fuelled concerns among Tory Euro-sceptics about the timetable for Brexit.
Privately, many backbenchers have serious misgivings about the plan for a two-year "implementation period" when most EU rules and regulations will continue to apply the UK after the official departure date in March 2019.
The Tory MP said: "The danger is that we are on course for nothing to have changed five years after we voted for Brexit.
"That gives an opportunity for all sorts of machinations and new offers to try to keep us back in.
"This is a real issue now. The Remainers are playing an effective game and we Brexiteers urgently need to up our game."
Tory MP Nigel Evans said: "Nothing should stop us leaving the EU. The time for an offer from Brussels was when David Cameron was trying to negotiate a new membership deal. He asked for little and got less.
"The British people have voted to entirely leave the EU and that is what we are going to do on March 29 2019. Their view has to be respected.
"Any attempt to give us some sort of 'EU-lite' membership is simply not going to work."
Richard Tice, co-chairman of the Brexit-supporting pressure group Leave Means Leave, said: "This shows a complete disregard for British democracy by European leaders.
"The British voters have chosen to leave the EU in its entirety and Britain becoming an associate member does not even come close.
"By delaying Brexit for two years, the PM has weakened Britain’s negotiating position and buoyed Macron and Merkel into thinking they can keep Britain shackled to the EU forever. The Government must unequivocally rule out this proposal."
President Macron’s speech follows European Commission President Jean-Claude Juncker’s recent “State of the Union” address to the European Parliament claiming that the dream of European unity was reawakening.
He insisted that the EU would rapidly step up integration after Britain left the bloc and declared: “The wind is back in Europe’s sails.”
And President Macron has put a drive to reform and renew the EU at the heart of the programme for his five-year presidency of France.
The centrist politician, leader of the new strongly pro-Brussels En Marche party, was elected to office in May after beating the nationalist and Euro-sceptic Marine Le Pen in the race to the Elysee Palace.
Since taking over, he has formed a firm alliance with Angela Merkel. It is thought that her expected return to office in Berlin will mean the pair will spearheaded a united effort towards European unity.