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.
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