In his first interview since resigning as work and pensions secretary Friday, Iain Duncan Smith accused Cameron of trying to reduce Britain's budget deficit through benefit cuts which hurt poorer voters while protecting older, often richer ones.
Duncan Smith, who last month became one of the most senior Conservatives to say he would campaign against the premier for Britain to leave the EU on June 23, denied his shock resignation was about Europe.
But the former army officer known as IDS who led the ruling party from 2001 to 2003 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".
Duncan Smith also said that Cameron's government was "in danger of drifting in a direction that divides society, not unites it".
The resignation of Duncan Smith 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 which Cameron admitted in an interview published Sunday would be close.
"My fear is turnout," the prime minister told the Independent on Sunday. "For heaven's sake, get out and vote in, because you might wake up and find you're out."
Inflicting 'maximum damage'?
Duncan Smith says he resigned over £1.3 billion (1.2 billion euros, $1.4 billion) of cuts to disability welfare announced in a budget last week which "benefits higher earning taxpayers" -- often Conservative voters.
But a furious Cameron called Duncan Smith a four-letter word in a phone call to discuss the resignation, describing him as "dishonourable", media reported.
And ministers in Cameron's government were arguing 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, said 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 issued statements supporting him.
Another Duncan Smith ally, senior eurosceptic Bernard Jenkin, complained of high-handed leadership by Cameron, telling Sky News that "the prime minister... is not meant to be a dictator".
David Laws, who served as a Liberal Democrat minister in the last coalition government under Cameron, said on the BBC that the implications of the current row were "huge".
"I hate to intrude into a civil war which is now dominating British politics," he added.
As the Duncan Smith row overshadowed the wider referendum debate, Conservative former prime minister John Major stepped forward to argue that a Britain outside the EU would be much less powerful that at present.
"Our nation can either decide to be true to our history -- and remain outward-looking internationalists on the world stage -- or shrink to lower prominence," he wrote in the Sunday Telegraph.
Major -- the last Conservative to lead Britain before Cameron, between 1990 and 1997 -- endured a turbulent term in office due to party infighting over the creation of the European Union.
He famously dubbed leading eurosceptics who plotted against him "bastards".
