Clare Short has launched a fresh assault on the prime minister and called on Gordon Brown to "make his move" for the leadership.

The former international development secretary says in an interview to be broadcast on Sunday that the chancellor will be "diminished" if he does not seek to dislodge Tony Blair soon.

She argued that Brown would lose credibility with the Labour Party rank and file if he was seen to be too loyal and close to Number 10 in the final few year's of Blair's leadership.

Although the prime minister has already announced he would not fight for a fourth term in office, Short says her former Cabinet ally cannot afford to wait that long.

While Brown remains a "credible and capable" she warns that his position as heir apparent will not last indefinitely.

"Lots of people are waiting for Gordon to make his move, and if he can't or won't, that sort of diminishes him as the alternative. But I understand he's on a tightrope. It's a very difficult position," she tells GMTV.

She also claims that a hung parliament is "on the edge of possibilities" in the election expected next year because of Blair's lack of popularity and stifling of debate within the party.

Iraq

Having resigned from the government over the issue of Iraq, even though she remained in post during the war, Short says too much power has been concentrated in the hands of Downing Street.

"It really is an elective dictatorship and I think that's very error-prone," she argues.

"The way the system runs, it's very ruthless; the patronage is very powerful. The way people get slated and briefed against if they step out of line is very nasty."

And while she draws back from accusing the prime minister of lying over the case for war, Short claims that the legal basis for the conflict was unsound and influenced by Blair.

"A lie is the intent to deceive. Now I think he thought that was the right thing to do. I don't think it was the right thing to do, either the deceit or his basic policy decisions," Short says.

But she alleges Blair pressured the attorney general Lord Goldsmith over the legal case without telling the Cabinet.

"He's a personal appointee of the prime minister, a friend of the prime minister. I think he was leant on by the prime minister," she says.