Leaving the European Union would potentially cost the average working Briton the equivalent of a month’s salaryby 2020, the Organisation for Economic Cooperation and Development said.

Angel Gurria, secretary general of the organisation said Britain would have less access to the bloc’s single market, and ‘Out’ campaigners were indulging in wishful thinking ahead of the country’s referendum on June 23.

Campaigners in the ‘Vote Leave’ camp dismissed the comments, saying the OECD’s credibility was damaged by its past promotion of the benefits of the euro, and the fact that it received some funding from the European Commission.

Support for the campaign to get Britain out of the European Union has risen in recent days, two opinion polls showed on Tuesday, in the face of calls from US President Barack Obama and other global figures for the UK to stay in the bloc.

However, the Financial Times poll of polls showed the ‘stay’ support up to 47 per cent, with the number of undecided going down over the past few weeks to 11 per cent from 13 per cent.

Barack Obama warned Britain would move to ‘the back of the queue’ in trade talks

Britain could see a hit to investment and would not have a better deal on the flows of migrant workers, he added.

“Out” campaigners, chief among them London Mayor Boris Johnson, argue that Britain’s economy would flourish outside the EU by saving its annual contributions to bloc, freeing itself of red tape and striking its own trade deals.

“After [we] Vote Leave and take back control we will be able to cut our tax bill because we will no longer have to fund overpaid and under-taxed international bureaucrats,” Vote Leave spokesman Robert Oxley said in response to Gurria’s comments.

Earlier this month the International Monetary Fund said Brexit would deal a damaging blow to the global economy.

And last week, US President Barack Obama warned Britain would move to “the back of the queue” in trade talks with Washington if it left the bloc.

The OECD’s warning of lost income to British workers echoed the message from Britain’s finance ministry which said last week that households would be £4,300 worse off each year by 2030 if the country left the EU than if it stayed.