Theresa May is being urged to change course and seek full membership of the European single market and customs union by 20 British MEPs, including three Tories and the majority of Labour politicians based in Brussels.
In a letter that lays down a challenge for the prime minister but also the Labour leader, Jeremy Corbyn, the group claims the case for staying in the internal market has become stronger since the referendum.
They warn that crashing out of the economic grouping would make Britain poorer and suggest that voters should still be given a chance to rethink Brexit altogether.
“The best way to secure Britain’s prosperity would be to remain close to Europe, inside the single market and customs union, and to secure a deal that keeps Britain in the room,” write the MEPs, who are supporters of the Open Britain campaign.
“Sadly, this no longer seems likely. So, if the price of a Brexit turns out to be a loss of control over the rules and an economy that will leave us poorer, people have every right to keep an open mind about whether the Brexit course chartered by our government is the right path for our country.”
They call it a “lamentable irony” that Britain helped shape the single market but that successive governments have failed to make the argument for its benefits.
Since the referendum, they highlight moves by the EU to tackle corporate tax avoidance by tech giants like Amazon, to save Brits money abroad by scrapping roaming charges, and new trade deals with countries such as Canada and Japan.
They also say free movement has been reformed to minimise the undercutting of wages.
“Leaving the EU means giving up our seat at Europe’s top table and risks making us a rule taker rather than a rule maker,” they add, arguing that single market membership would maintain most influence.
As well as some Conservatives, Lib Dems and Greens, the letter includes 12 of Labour’s 20 MEPs, meaning a majority of the party’s European delegation are challenging party policy in the UK.
Keir Starmer, the shadow Brexit secretary, has said Labour wants the UK to stay inside the single market and customs union during transition, but has only promised to try to replicate the benefits of membership afterwards.
The party is grappling with the challenge of representing many constituencies which voted to leave the EU while having more than 80% of members who want to retain membership of both the single market and customs union.
Meanwhile, the former Labour prime minister, Tony Blair, claimed that the party could become the “handmaiden of Brexit” if it failed to take a firmer stance.
However, sources within the party pointed to recent comments by senior figures such as the shadow chancellor, John McDonnell, who said “we want to be as close to the single market as we possibly can”.
Clare Moody, a Labour MEP for the south-west and Gibraltar, said her party’s position on Brexit “has moved and we will see how much further it moves in 2018”.
“The key principle is that we do not want the British public to be worse off as a result of negotiations – that is fundamental,” she added, arguing that Labour should not “close off avenues” as the Tories had done.
Charles Tannock, one of the Conservative MEPs who signed the letter, described the 52% victory for leave in the EU referendum as a margin “not convincing for Brexit, let alone the hardest of Brexits” given the scale of constitutional change. He argued that other countries would demand a super majority for something so critical.
He also said that the result was based on a “strange election franchise” in which Commonwealth country citizens were allowed to vote even if unaffected by the outcome but his eastern European wife could not, and nor could his British-born mother who had lived for more than 15 years in Paris.
“There is no clear mandate for a hard Brexit so I believe we should have tried to stay inside the single market and customs union for five years and then decide what we want – to re-enter as a full EU member or, if it becomes available, a future associate membership or opt for hard Brexit,” he said.
Tannock, a London MEP who admitted he was in a minority within his party, said the government had instead taken a “total separation” view by insisting “Brexit means Brexit”.
His two colleagues, Julie Girling and Richard Ashworth, who have also signed the letter, remain members of the Conservative party but have had the whip withdrawn over a recent vote in Brussels.
A UK government spokesman said: “We respect the position of European leaders that the four freedoms of the single market are indivisible. As the prime minister set out in her Lancaster House speech, we will be leaving the single market and the customs union, taking control of our borders and ending the jurisdiction of the ECJ.”
He argued that single-market membership outside the EU would force Britain to accept rules over which it would have “little influence and no vote”. “Such a loss of democratic control could not work for the British people.”
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