David Cameron has warned Gordon Brown lacks a "full-throated mandate" to take over as prime minister later this year.
The Conservative leader said there must be a general election when Tony Blair steps down, rather than a coronation of the chancellor as expected.
"Whoever takes over must know they haven't got that full-throated mandate from the British people, so we should have an early election," he told BBC One's Sunday AM.
Mr Cameron attempted to draw a distinction between the current situation and John Major taking over from Margaret Thatcher in 1990.
"I think there's a difference this time in that Tony Blair uniquely said before the last election that 'I'm not going to fight another election but I'm going to do a full term'," he said.
Although he said there was no doubt Mr Brown is a "formidable politician", the Tory leader also criticised his likely opponent in the polls next year as having a "problem" with his record on state control.
But he admitted he "does not really know" the chancellor personally, saying: "We have only met and talked a couple of times. Sometimes in politics you have some friendships across the floor of the Commons but I just don't really know him."
In a speech yesterday that further set out his credentials to be Britain's next prime minister, Mr Brown reacted to criticism that he lacked charisma by claiming genuine "character and personality" was maintaining your "beliefs and values".
The chancellor also accused those calling for Scottish independence of playing "fast and loose" with the historical political union of the UK's constituent parts.
Mr Cameron however today laid the blame firmly at Labour's door, accusing their devolutionary efforts of "carving [Britain] up".