Anna Soubry and other leading Tories join Labour, Lib Dems and SNP in cross-party bid to keep UK in the customs union
The Remoaner proposal would prevent Britain from ever signing new trade deals with the like of the US or Japan.
SOFT-Brexit Tories have signalled they will rebel to force the UK to stay in the customs union, giving Theresa May a new headache.
Conservative MPs – including ex-minister Anna Soubry – have signed up to a cross-party group of MPs demanding the UK permanently stays in the union, which removes tariffs and barriers from trade.
It would bar Britain from ever signing new trade deals – a key argument Leave campaigners promoted in the referendum campaign.
It would also put Liam Fox out of a job as International Trade Secretary.
But Labour says it would look at staying in the EU's customs union indefinitely.
Tory Brexit supporters fear their Europhile colleagues would exploit the PM's slim majority and join Labour, the Lib Dems and the SNP to deliver an embarrassing defeat in a Commons vote on the issue.
The report - signed by Labour Europhile Chuka Umunna and backed by Lib Dem and SNP MPs - claims leaving the customs union would be "reckless and economically dangerous".
They said making an "ideological choice" to "wrench" Britain out of the free trade group would "unilaterally surrender the best economic option for our country".
The report - titled The Case for Continued Customs Union Membership - states that the cost of leaving the customs union would be an estimated £25 billion annually.
It adds that only through continued membership of the customs union can frictionless trade between the UK and EU continue.
The group issued 10 challenges to the Government, asking them to reveal how they would deliver the same trade benefits outside the customs union and how they could make a new customs union work before Britain left the EU.
They also asked the Government to "provide greater clarity" about how it intended to "negotiate and sign new trade deals while still operating within an interim customs arrangement with the EU".
In a foreword, Mr Umunna and Ms Soubry said: "Leaving the customs union would be a reckless and economically dangerous self-inflicted wound. It doesn't have to be that way.
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"Ministers have shown greater pragmatism on several Brexit issues in recent weeks, which is welcome.
"But on the issue of customs, rhetoric simply does not match reality.
"Their hasty choice to leave the customs union, and their lack of realism and preparation regarding real alternatives, increases the chances that we could face a crash into chaos and confusion in our customs system after Brexit."
They said new proposals put forward for leaving the customs union "look nothing short of a Brexit bureaucracy bombshell for British businesses", despite cuts to red tape being one of the purported benefits of leaving the EU.
"In our view, a total commitment to full membership of the customs union is what is required in the national interest, not just for a transitional period but for the long-term future," they added.