Leaving the EU would cost the average working Briton the equivalent of a month’s salary by 2020, according to an OECD report due out later on Wednesday which was cited by BBC radio.

Angel Gurria, secretary general of the organization which groups many of the world’s leading economies, said Britain would not get a better deal for its economy outside the EU than in it, and he said “Out” campaigners were indulging in wishful thinking ahead of the country’s referendum on June 23.

“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.

“There is no kind of deal that could go better by yourselves than you would be in the company of the Europeans.”

“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.

Opinion polls show Britons are relatively evenly split on whether to leave the EU, despite warnings from international economic bodies that Brexit would leave them worse off.

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

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