Former British prime minister John Major yesterday launched a blistering attack on fellow Conservatives campaigning for a Brexit, as rivals traded economic claims in an increasingly tight race for the June 23 vote.

Major condemned the "deceitful" campaign by Vote Leave, the Conservative-led group pressing for Britain to leave the European Union, saying it was making "fundamentally dishonest" claims about the potential economic consequences.

His intervention followed an accusation by Boris Johnson, the former Conservative mayor of London, that voters "cannot trust" government promises that Britain would not be forced to contribute to any future eurozone bailouts.

There are less than three weeks left in a race that opinion polls show is

on a knife edge, raising the prospect of Britain becoming the first country to leave the 28-nation bloc.

The average of the last six polls calculated by the What UK Thinks research project yesterday put both sides at 50 percent, excluding undecided voters. The "Remain" camp previously had a slight lead.

The economic consequences of staying in or leaving the EU is one of the key battlegrounds of the campaign, which is being fought between two main groups, both dominated by members of Prime Minister David Cameron's Conservatives.