The leader of Britain's biggest union has said Tony Blair should stand down as prime minister.

The Unison general secretary's comments came as new reports suggest growing divisions over health policy between the prime minister and Gordon Brown.

Dave Prentis said on Tuesday the chancellor should get his chance sooner rather than later and that a timetable should be set for the handover.

The move will re-open the rift between Blair and Brown that has become more public in recent weeks.

Although the comments were unlikely to have been sanctioned by Brown and could have been motivated by the fact that Prentis is facing re-election next week, they show the extent of the divide in the labour movement.

The public service workers' chief said Brown has been an "excellent chancellor" and would be the best man to "renew" the government.

He also suggested recent accounts of Blair going back on deal to step down last year showed the prime minister has been "dishonourable" towards his colleague.

"There comes a time when there will be a change of leader," he told the Daily Record.

"And it is right we do renew ourselves. I think arrangements were made for when Tony Blair may well think of stepping down, and I think something dishonourable is happening at the moment.

"I don't know if he has reneged or what. But we do need a real timetable for change over."

NHS reform

The comments came as a new biography of Brown suggests the prime minister and chancellor are fundamentally at odds over the future of the NHS.

Robert Peston's "Brown's Britain" quotes a Treasury adviser saying the chancellor believes Blair and Labour's policy chief Alan Milburn see the health service as an "organising idea" rather than a provider of care.

The dispute could lead to a rift over Labour's election manifesto, which Blair has promised will be "unremittingly New Labour" and "market based".

Prentis backs Brown's reported position that markets are not an appropriate means of distributing healthcare resources.