Leaving EU would cost Britons a month's salary by 2020

Leaving the European Union would cost the average working Briton the equivalent of a month's salary by 2020, the Organisation for Economic Co-operation and Development said on Wednesday, joining a list of economic bodies warning against an exit.

Angel Gurria, secretary general of the organisation which groups many of the world's leading economies, 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 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 U.S. President Barack Obama and other global figures for the UK to stay in the bloc.

"There is no kind of deal that could go better by yourselves than you would be in the company of the Europeans," Gurria said before the release of an OECD report on Brexit later on Wednesday.

"WISHFUL THINKING"

"We made a whole series of calculations and we came out saying Brexit is a tax ... It's equivalent to roughly missing on one month's income within four years and then it carries on ... and there's a consistent loss," he told the BBC.

"This is not wishful thinking, which we believe that the Brexit camp has in many cases been assuming on a number of things that, you know, could go in their way," he said.

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.

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 pounds ($6,281) worse off each year by 2030 if the country left the EU than if it stayed.

($1 = 0.6846 pounds)