Nine out of 10 of Britain's top economists working in London's City financial district, small business and academia believe the economy will be harmed if Britain leaves the European Union, a poll said on Sunday.

The Observer newspaper said the poll was the biggest of its kind, drawing responses from more than 600 economists. It is a boost for Prime Minister David Cameron, who is leading the campaign for Britain to stay in the 28-member bloc at a referendum on June 23 despite opposition from some in his party.

Carried out by pollster Ipsos-MORI, the poll found that 88 per cent of those asked said an exit from the EU and the single market would probably damage Britain's growth prospects over the next five years and 82 per cent said there would probably be a negative impact on household incomes.

The Observer said those surveyed were members of the profession's most respected representative bodies, the Royal Economic Society and the Society of Business Economists.

Campaigners on both sides of the argument have targeted the economy as one of the main battle grounds to win round voters who are split in what is becoming an increasingly bitter fight over Britain's future.

'Leave' campaigners say Britain would be freed from regulation and red tape if it left the European Union, able to negotiate its own trade deals without having to please 27 other countries.

Matthew Elliott, chief executive of Vote Leave, accused economists of making "the same hollow arguments" they made to support switching to the euro.

"There was a cosy consensus among economists supporting Britain scrapping the pound 15 years ago...They were wrong then and they are wrong now," he said in a statement.

But the 'Remain' campaign has pressed its argument that Britain would suffer an economic downturn if it left, hurting the pound, jobs and wages.

Cameron said in a statement: "This poll confirms the overwhelming view of economists – leaving the EU would damage our economy, costing jobs and increasing prices."