The cross-party group, which includes representatives of the Conservatives, Labour, the SNP, Greens and Liberal Democrats, expressed “deep concern” over the Government’s current Brexit strategy, which will see Britain leave the single market and customs union after a transitional period.
The case for staying in the trading bloc has grown stronger since last year’s Brexit vote, they said.
In an open letter, the MEPs suggested people were right to question “whether the Brexit course chartered by our Government is the right path for our country”.
The letter was signed by 12 Labour MEPs, including the party’s deputy leader in the European Parliament, and three Conservatives, including the Tory group’s foreign affairs spokesman. Two SNP representatives, two Greens and one Liberal Democrat also backed the call.
It puts the Conservative and Labour MEPs on a collision course with their party leaders and raises the prospect of them being forced to give up the party whip.
Two Conservative MEPs, Julie Girling and Richard Ashworth, had the whip removed in October after they voted in support of a motion saying “sufficient progress” had not been made in Brexit negotiations. Both signed the new letter, which was published by the Open Britain campaign.
And Alex Cunningham, a Labour shadow pensions minister in Westminster, resigned this week after defying the party leadership in order to vote in favour of keeping the UK in the customs union.
In their letter, the MEPs wrote: “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.
“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.
“Sadly, this no longer seems likely. So, if the price of 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 said it was a “lamentable irony” that, having helped create the single market, successive UK governments have “so singularly failed to explain to the public the many benefits it has brought to our country”.
Highlighting a number of EU initiatives that have been introduced since last year’s Brexit vote, the cross-party group said: “Few in the UK will have noticed but in the year and a half since the referendum, some major and valuable changes have taken place in how the largest free trade area in the world works.”
Moves to tackle avoidance by tech giants, stop the undercutting of wages and reduce scrap mobile phone roaming costs in Europe were just some of the benefits to the UK of staying in the single market, they said.
The Government believes leaving the single market is crucial to reducing immigration and removing the UK from the jurisdiction of the European Court of Justice.
In her landmark Brexit speech in Florence last year, Ms May said: “The United Kingdom is leaving the European Union. We will no longer be members of its single market or its customs union. For we understand that the single market’s four freedoms are indivisible for our European friends.”
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