Labour's Shadow Trade Secretary Barry Gardiner is reportedly preparing to join the race for Labour leader.
According to HuffPost, the Corbyn ally is considering a tilt at the top job.
But in a series of tweets Unite boss Len McCluskey denied having any conversation with Mr Gardiner about the leadership race and said the MP wasn't considering a run.
In a tweet the Unite boss said : "I have just spoken to @BarryGardiner who is in Abu Dhabi. Barry is as surprised as I am by this
"We're both clear that fake news and tricks like this will not be allowed to undermine any candidate's campaign."
If Mr Gardiner is standing he has until Monday January 22 to secure nominations.
The late addition to race could dismay around the camps of other candidates already fighting to get nominations from MPs to proceed to the next stage of the contest.
Mr Gardiner, who is seen as a loyal ally of Jeremy Corbyn and was one few trusted by his his team to make media appearances.
A source reportedly close to Gardiner told HuffPost: “Barry recognises the challenges the party faces over the next five years and believes he has the broad base of support, experience and loyalty to the party to win the race and take the fight to the Tories.”
It comes just hours after Mr Starmer stormed into the lead in the Labour leadership race with 23 MP nominations on the first day of canvassing in the Parliamentary Labour Party.
It means the Shadow Brexit Secretary is the first candidate to get over the line with the 41 nominations needed to get on the ballot paper.
Left-wing rival Rebecca Long Bailey picked up high profile endorsements from Diane Abbott and John McDonnell.
On the first day, Corbyn allies' favourite Rebecca Long-Bailey was second in MP's nominations with 17 nominations.
Jess Phillips has 16, Lisa Nandy had 11, Emily Thornberry had 3 - while Clive Lewis had one.
