Some Labour Party members have not received their ballot papers to vote for the party's new leader, one of the contenders has said.
Hackney MP Diane Abbott said she had been told many members in London have not received the ballots even though voting closed at 1700 BST on Wednesday.
Ex-ministers David and Ed Miliband, Ed Balls and Andy Burnham are the other leadership candidates.
Party members are also voting for Labour's 2012 London mayoral candidate.
Former mayor Ken Livingstone and former MP Oona King are battling for the candidacy.
The Labour leadership results are due on Saturday, with the party's mayoral candidate to be announced on Friday.
Speaking to BBC London 94.9, Ms Abbott said: "A lot of people, particularly in London, have said they have not had their ballots yet.
"So I am going to take this up with Labour Party headquarters this morning because it is obviously a problem."
Ms Abbott, who said she was "quietly confident" in the leadership election, said people could not vote online either as they need a ballot number which they have not got yet.
A Labour Party spokesperson said: "The leadership election has been an exciting contest which has involved party members up and down the country and brought thousands of new members into the Labour Party.
"Millions of ballot papers have been issued and whenever problems have been reported we have taken action to solve them."