Trevor Mallard has reportedly sent an email to his fellow MPs saying he would not stay silent on the Labour leadership race.

RadioLIVE reported that Labour Party senior whip Chris Hipkins told MPs to not publicly discuss who they were supporting in the leadership race.

Mr Mallard sent an email saying he didn't want to be gagged by Mr Hipkins, and wanted to tell the public who he backed, RadioLIVE reported.

However, Labour president Moira Coatsworth told the radio station that there were no gagging orders on MPs.

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Wellington Central MP Grant Robertson declared his intention to seek the leadership when David Cunliffe gave notice on Saturday that he would resign tomorrow and seek a new mandate.

Deputy leader David Parker is expected to become acting leader.

Mr Robertson and Mr Cunliffe were the main rivals in last year's leadership contest, which was forced by the resignation of David Shearer who believed he was about to lose a no-confidence motion.

Labour has now been through four leaders while Prime Minister John Key has led National -- Helen Clark, Phil Goff, David Shearer and David Cunliffe.

Mr Robertson said the party needed a new generation at the helm.

He will consider going to the leadership contest on a ticket -- naming who he would like to see as his deputy.

"Clearly that would give further shape to the kind of leadership that I would bring."

He agreed Jacinda Ardern was a possibility but no decision had been made and it was ultimately a caucus decision.

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"What I am clear about is I think we need a new generation of leadership. That's the generational change that we require now to meet the new challenges that the party has got.

"I've got to take some time to sit down and listen to my colleagues, along with everybody else."