Business Secretary Sajid Javid warned that leaving the European Union could cost British businesses 34.4 billion pounds ($49.7 billion) in “non-tariff barriers” to trade as a result of being outside the single market. The claim, made in a letter co-signed with former Business Secretary Peter Mandelson, a member of the opposition Labour Party, was immediately dismissed by the “Leave” campaign, which accused Javid of using “random numbers” to back Prime Minister David Cameron’s campaign to stay in the 28-nation bloc. “Inside the single market we can guarantee continued growth in employment, greater opportunities for our young people, higher investment in our public services and new trade agreements with the world’s global powers,” Javid and Mandelson wrote. “U.K. businesses could face a 34 billion pound export tax if we leave, which would hit jobs and growth, with each exporting business facing on average £80,000 in additional costs.” With surveys by Britain’s discredited polling industry fluctuating between a win for either side in the June 23 referendum, the campaign to stay in the EU is seeking to reinforce its economic case, which has been backed by the International Monetary Fund and the World Bank. Those arguing that Britain would be better off out are focusing on the loss of control over immigration and the erosion of sovereignty as a result of membership of the bloc. An online poll by YouGov Plc published in Tuesday’s Times newspaper showed ‘Remain’ at 43 percent and ‘Leave’ at 42 percent. The pound fell on Monday when a survey published by the same pollster had ‘Leave’ in the lead at 45 percent and ‘remain’ at 41 percent. “With momentum shifting in favor of leaving the EU we are seeing yet more random numbers from the remain camp,” James Cleverly, a lawmaker from Cameron’s Conservative Party, said in an e-mailed statement responding to the 34 billion pound claim. “The remain campaign are failing to make the positive case for the EU because there isn’t one,” he said. “The costs of the EU still massively outweigh the benefits.” Hiroaki Nakanishi, chairman of Hitachi Ltd, said Tuesday the Japanese manufacturing giant would “rethink” its activities in the U.K. if Britain votes to leave the EU. “Brexit would force us and similar companies to rethink, because we still have a European vision, and would be disadvantaged in pursuing it from the U.K.,” Nakanishi wrote in an article for the Daily Mirror newspaper. “We understand the EU is not perfect, but we hope the U.K. can continue from inside to change it for the better.” The “Britain Stronger in Europe” campaign group, which produced the estimate of how much additional trade barriers would cost British business, said it had based its calculations on the charges faced by U.S. businesses as a result of being outside the European Customs Union and the single market. “Non-tariff barriers would take the form of having to meet rules and regulations required in the destination country over, for example, submitting customs declarations; accepting delays while waiting for products to clear inspections; adapting products to differing specifications and standards; needing new licenses, authorizations or qualifications,” the group said in a briefing note. The U.S. example is “likely to be the upper range of the non-tariff barriers exporters would face,” the note conceded, but maintained it is “realistic as it is based on options advocated by leading ‘Vote Leave’ campaigners.” Javid and Mandelson’s letter, addressed to “Vote Leave,” asked for details of the effects on British business of the Norwegian, Swiss, Albanian and Canadian models for trade with the EU. “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,” they wrote. “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.” Former London Mayor Boris Johnson, a leading campaigner to quit the bloc, said Cameron’s campaign to stay in has also failed to outline its plan for the future. If Britain stays in the EU, it won’t be able to control immigration and provide for its growing population, he said. “With the population growing by the size of the City of Newcastle every year, what is their plan for public services, for housing, for the health service, how are they going to pay for all this and how will the economy cope?” he asked in a speech in Stratford-upon-Avon on Monday. “The risks of remaining in this over centralised, over regulated job destroying machine are becoming more and more obvious and that’s why we’re winning the argument.”