text: Last week Mr Cameron told the BBC he would not serve a third term as PM.
  Asked by Andrew Marr if that meant Mr Cameron would have to "stand down at some point during the next Parliament", Mr Duncan Smith replied: "He does."
  Downing Street said the PM would serve "every single day" of a second term.
  In his BBC interview on Monday, the prime minister said that, if re-elected this May, he would serve the full five years of another Parliament and then leave Number 10 - leading Labour to accuse him of arrogance.
  Mr Duncan Smith told BBC One's Andrew Marr Show the "way that all comes about will have to be discussed" adding that Mr Cameron would "serve what essentially is a full term" - but there would "of course" be a leadership contest at some point.
  "You've had to literally rap the knuckles of people like Gordon Brown and previous prime ministers to get them to think of going," he said.
  "[Mr Cameron] is actually very keen to say 'there is a limit, there's an amount of time a prime minister should serve before they get stale,' and he is right about that."
  Mr Cameron has tipped Home Secretary Theresa May, Chancellor George Osborne and London Mayor Boris Johnson as potential successors.
  But Mr Duncan Smith said that, having been Conservative leader at a time when the party "was quite interested in having a fight in an empty room", he would not make predictions.
  He said he expected the next Conservative leader would already have been "in the public sphere for some time".
  Following Sunday's interview, a Downing Street source told the BBC: "David is very clear he will serve a full five-year second term, every single day of that as prime minister.
  "In terms of what happens in five years' time, we'll sort that out in five years' time."
  A YouGov poll for the Sunday Times  has suggested Labour has a four-point lead over the Conservatives, putting Labour on 36% with the Tories on 32% - while an Opinium poll for The Observer suggests the Tories are on 34% and Labour are on 33%.
target: The Conservatives would have to choose a new leader before the end of the next Parliament if David Cameron wins a second term as prime minister, Welfare Secretary Iain Duncan Smith has said.