The leader of Britain's biggest union wants a date to be set for Tony Blair to be replaced as Labour leader.
Dave Prentis said it was right to change leader
Dave Prentis, general secretary of Unison, also questioned whether Mr Blair had reneged on a deal to let Chancellor Gordon Brown take over.
Mr Prentis told Scotland's Daily Record newspaper: "It is right we do renew ourselves. We do need a real timetable for changeover."
The prime minister has rejected reports that he told Mr Brown he would quit.
Mr Prentis, who believes Mr Brown is an "excellent chancellor", said: "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."
Mr Prentis added: "There comes a time when there will be a change of leader. And it is right we do renew ourselves."
The prime minister insisted last week that no arrangement had been made, saying: "You don't do deals over jobs like this".
The Daily Record carried out a poll which suggested that 60% of Scots would vote Labour if Brown was prime minister, compared to 48% if Mr Blair was in charge.