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.