The prime minister will hold face-to-face talks with the US President at the White House on Friday – the first foreign leader to do so since his inauguration.
Mrs May called on the US to renew its “special relationship” with Britain as she looks to set the ground for a trade deal with the world’s largest economy ahead of the hour-long meeting at the White House.
Any agreement would come as a blow to EU bureaucrats, who fear the UK’s talks with Mr Trump may give the prime minister leverage in the forth coming Brexit negotiations.
Axel Schafer said any relationship between the UK and US was "illusional"
The European unification has been a big success for Britain since 1973
Following the announcement, a senior member of Germany's Social Democrat Party, Mr's Merkel's coalition partners, has been forced to play down Britain's optimistic future with the US – branding the relationship "illusional".
Axel Schafer, a key Brexit adviser to the EU, warned Britain was in a “lose, lose” situation by ditching the bloc in a bid to go global.
“I think it’s a big illusion – the European unification has been a big success for Britain since 1973,” he told BBC Radio 4’s Today programme.
Theresa May is in the US as the first foreign leader to meet Donald Trump
The prime minister has promised to ditch the UK’s membership to the EU’s single market and instead create a new trading relationship with the bloc.
He continued: “Now Mrs May thinks she can have a Brexit that is something like a divorce but still spending holidays together – it just does not work.”
The Europhile suggested Britain faced isolation from the remaining 27 member states once the divorce had been completed, claiming Brussels held all of trump negotiating cards.
“Theresa May is not in a position of strength, she is under a lot of pressure," he continued.
“All Europeans, from Social Democrats, Conservatives, Christian Democrats, Liberals and Green parties all over Europe believe it would be a lose-lose situation and not a win-win situation for Britain.”
Any trade deal talks will come as a blow to Brussels ahead of Brexit talks
President Trump previously suggested more countries would follow Britain’s lead and vote to quit the EU – hailing Brexit a “great thing”.
Playing down the claims, Mr Schafer said the remaining 27 countries were fully committed to the Brussels bloc – with support in Germany as strong as it has ever been.
“These are all illusions – in my Parliament, there are 631 MPs, we are all in favour of a Remain situation so maybe you can imagine why it is sad for us that the British people have made this decision,” he said.
May meets Trump: Historic first meeting in pictures
Fri, January 27, 2017
The two leaders will spend about an hour in face-to-face talks in the Oval Office, where President Trump has restored a bust of Winston Churchill removed by predecessor Barack Obama.
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Prime Minister Theresa May speaks during the 2017
The prime minister spoke to Republican in Philadelphia on Thursday
As she arrived in the US on Thursday, Mrs May delivered a rallying cry to US Republicans in Philadelphia that Britain would succeed post-Brexit.
She said: "People voted to leave the EU with determination and quiet resolve.
"We can now build links with the world and our friends in the EU. We're not turning our back on Europe and will continue to cooperate on trade and security and it's in Britain's interests that the EU succeeds.
"But it gives us an opportunity for self-determination and to become a more global and internationalist nation, to take back control of our borders and immigration and to determine our own laws."