Ministers are being urged to ditch the "fallacy" of securing easy trade deals after Brexit amid fears the UK will be forced to accept reduced standards as well as tough trade-offs on visas and tariffs.
Ahead of a major Commons battle over Brexit next week, pro-EU campaigners will publish detailed analysis of Britain's future trading prospects with major global players such as India and China.
Senior Remain-backing MPs will warn that Britain risks "crippling" its reputation as a top trading nation if it leaves the EU's customs union and single market and is forced to surrender on trade terms to powerful allies like the United States.
Liam Fox, the international trade secretary, previously argued that securing a trade deal with the EU would be the "easiest in human history" but critics have cast doubt on his optimism about securing new deals for Britain for the first time in decades.
The intervention comes as it emerged that Theresa May will seek to overturn all 15 amendments to her flagship Brexit legislation in a single day, including key battlegrounds such as the Irish border, and leaving the customs union and the single market.
Launching the Open Britain report on Wednesday, Tory backbencher, Anna Soubry, will say: “It's a fallacy that we're better off out chasing unicorn deals on our own and that's recognised by the government.
"Its own assessments show even if we got a deal with every available country it wouldn't make good the harm we will do by walking out of the customs union and putting up custom barriers with the EU."
Australia and American negotiators will put pressure of the UK to drive down standards in food and farming, opening the door for products that are currently banned in the EU such as chlorine-washed chicken and hormone-fed beef, according the paper.
Both China and the US are traditionally reluctant to open up their services sectors to overseas workers, limiting the benefit for British expats who want to work in areas such as hospitality, the report found.
In the wake of Donald Trump's decision to slap punitive tariffs on US steel imports, the group said changes to tariffs could price British producers out of tight markets, adding that a possible deal with New Zealand for lamb could have a major impact on Cumbrian and Welsh farmers.
Liberal Democrat leader Sir Vince Cable will warn that trade negotiations are "nasty, brutish and long" and claimed the government had made no attempt to explain the implications to the public.
“The notion that a raft of new deals can be done swiftly or bring overall benefit to the UK economy has now been exposed as a fantasy," the former business secretary said.
"And with President Trump imposing swingeing tariffs on our steel, the delusion that his administration will come to our rescue can shortly now be laid to rest.”
His concern is set to be echoed by Labour's Chuka Umunna, who urged his own party to stop "parroting Tory Brexiteers excuses" for keeping the UK in the European Economic Area (EEA).
Mr Umunna is due to say: "Next week crucial votes will take place on our future relationship with the European Union and they offer the chance to defeat a hard Brexit once-and-for-all.
“I know a growing number of my colleagues on the Labour backbenches are determined to use this opportunity to promote social justice, protect our trade, jobs and investment, and to safeguard peace and stability in Ireland, by backing all the Lords’ amendments.”
Antonia Romeo, the top civil servant at the Department for International Trade (DIT), previously acknowledged Britain could suffer a “loss of trade” with around 40 countries on Brexit day.
However a DIT spokesperson later added to her remarks, saying rolling over trade deals was a technical exercise and high level talks have been held with around 70 countries.
A Department for International Trade spokesperson said: “As we leave the EU we will create a future independent UK trade policy which supports the whole of the country. And, as the Prime Minister has said, Britain needs to be free outside of a customs union to take advantages of the opportunities of the fastest growing global markets.
"The IMF estimates, in the next 10 to 15 years, 90 per cent of global economic growth will originate from outside the European Union.
“Many countries have repeatedly made clear that they want to secure new agreements with the UK which will drive growth and deliver strong benefits for consumers and businesses alike.
“While we are committed to mutually beneficial economic trading arrangement with new partners, we have made very clear that the UK will maintain its strong consumer, food and animal welfare standards in all trade agreements the UK is party to.”
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