A top British eurosceptic minister who quit over welfare cuts launched a damaging attack on Prime Minister David Cameron Sunday, exposing tensions within his government ahead of June's referendum on EU membership.

In his first interview since resigning Friday, Iain Duncan Smith accused Cameron of trying to reduce Britain's budget deficit through benefit cuts which were unfairly hurting poorer voters while protecting older, richer ones.

Duncan Smith, who last month became one of the most prominent Conservatives to say he would campaign against Cameron for Britain to leave the EU on June 23, denied his shock resignation was about Europe.

But he admitted that Cameron and his finance minister and close ally George Osborne had stopped listening to him.

"This is not some secondary attempt to attack the prime minister or about Europe," Duncan Smith said in a BBC television interview, adding he quit because he was "losing that ability to influence events from the inside".

The resignation of Duncan Smith -- a former Conservative leader often referred to simply by his three initials, IDS -- is perhaps the biggest blow Cameron has suffered since being re-elected last year.

It comes just three months ahead of the referendum on EU membership on June 23 which opinion polls suggest will be closely fought.

In his resignation letter, he questioned whether Cameron was honouring his slogan that Britons were "all in this together", despite deep austerity cuts including £1.3 billion (1.2 billion euros, $1.4 billion) axed from annual disability welfare.

Meanwhile, a furious Cameron called Duncan Smith a four-letter word in a phone call to discuss the resignation, describing him as "dishonourable", the Mail on Sunday reported.

Ministers in Cameron's government argued publicly Sunday about whether Duncan Smith's resignation was a principled stand against benefit cuts or a eurosceptic plot to undermine the premier.

Pensions Minister Ros Altmann, who served under Duncan Smith, released a statement saying she was baffled by his decision to quit after Downing Street had already said it would rethink the cuts he was objecting to.

"He seems to want to do maximum damage to the party leadership in order to further his campaign to try to get Britain to leave the EU," she added.

In response, three other ministers who worked closely with Duncan Smith, including another leading eurosceptic, Priti Patel, issued statements supporting him.

David Laws, who served as a Liberal Democrat minister in the last coalition government under Cameron, told the BBC the implications of the current row were "huge".

"I hate to intrude into a civil war which is now dominating British politics," he added.