British firms could face a £34.4 billion "export tax" on trade with the European Union if the country votes to Leave, Business Secretary Sajid Javid has warned.
The Cabinet minister and former EU trade commissioner Lord Mandelson challenged Brexit campaigners to set out their plans for the UK's trading relationship, claiming that leaving the single market without an alternative would be an "act of economic sabotage".
Senior Tory Mr Javid and Labour grandee Lord Mandelson highlighted analysis by the Remain camp suggesting non-tariff barriers, such as complying with customs rules and regulations in destination countries, will cost exporters an average of £80,000 each.
In the letter to Vote Leave, Mr Javid and Lord Mandelson said: "After repeatedly claiming that Britain will be better off outside of the EU it is now incumbent on you to provide a detailed plan for Britain's future outside Europe.
"If you want to secure the trust of the British people important economic questions must be answered."
They added: "The economic damage of leaving is now clear. UK businesses could face a £34 billion export tax if we leave, which would hit jobs and growth, with each exporting business facing on average £80,000 in additional costs.
"This follows evidence which shows our trade could fall by £200 billion and up to 820,000 jobs could be lost."
The pair challenged the Brexit camp to spell out the future trade relation the UK would have: "The British people deserve answers to these questions, so they make a choice with their eyes open with clear facts, so they know exactly what they're getting.
"A campaign to leave the EU's single market without a plan for an alternative is an act of economic sabotage which would risk thousands of jobs, billions of trade and investment and the future economic stability of our country."
The new Britain Stronger In Europe analysis found that leaving the EU's single market would create a £34.4 billion "export tax" from the erection of non-tariff barriers on UK businesses that export to the EU. This is equivalent to an average £79,500 for each of the 430,000 British businesses that export to the EU.
The analysis was peer-reviewed by Tony Dolphin, independent economic consultant and formerly senior economist at the Institute for Public Policy Research. It has also been supported by Vicky Pryce, former head of the Government Economic Service.
Brexit-backing Tory MP James Cleverly said: "The Remain campaign are failing to make the positive case for the EU because there isn't one. With momentum shifting in favour of leaving the EU we are seeing yet more random numbers from the Remain camp.
"The costs of the EU still massively outweigh the benefits and the renegotiation didn't make a dent on cost of the Brussels bureaucracy. If we vote Leave we can take back control of our democracy, borders and economy and protect our prosperity for future generations."