The Tory leader slipped out the word 't***' as he explained why he did not use the Twitter social networking service.
He then risked making the situation worse by stating that the public was 'p*****' off' with politicians.
Mr Cameron quickly added: 'Sorry, I can't say that in the morning.'
He made the comments during an interview with Absolute Radio about quirky details of his life and character.
Asked whether he used Twitter, Mr Cameron said: 'The trouble with Twitter, the instantness of it - too many twits might make a t***.'
The remark was greeted with laughter in the studio, with host Christian O'Connell saying: 'That's fantastic.'
More...
Shortly afterwards, the Tory leader was talking about how focused he was on the forthcoming general election, and the effects of the expenses scandal.
He said: 'The public are rightly, I think, p****** off - sorry I can't say that in the morning - angry with politicians.'
Aides stressed that Mr Cameron had apologised immediately for the latter slip, and pointed out that 't***' was not defined as a swear word under radio guidelines.
The interview is part of a fresh bid by the Tories to appeal to younger voters and destroy the view the party is one for the older generation.
Mr Cameron famously said in a radio interview three years ago that 'lots of people call me Dave' - creating a moniker that has stayed with him.
Today, he revealed he still asked himself whether he was the right person to become Prime Minister and lead the country.
'Of course you think all the time about "am I going to be able to take those really big decisions, take that responsibility, suffer the consequences when things go wrong, take difficult decision that could involve sending people into harm's way?", he said.
'I did think about that, though, before standing for the leadership of the Conservative Party, because I always thought there was a good chance of winning that.
'I thought very carefully "am I able to make those decisions, am I tough enough, am I strong enough to do it?".
'And I thought "yes I want to do this, I can do this and I am going to give it my best shot".'