Ex-Labour leader Neil Kinnock remains "fundamentally and irretrievably" against Ken Livingstone being allowed back into the Labour Party.
Mr Livingstone was expelled for running as an independent in 2000
Mr Kinnock told BBC Radio 4's Today programme the London mayor could not be trusted to uphold the party's aims.
Labour's ruling board is expected to decide this week whether to readmit Mr Livingstone to the party early.
He was expelled for five years for running against Labour in the 2000 mayoral poll.
Mr Kinnock, now the vice-president of the European Commission, said: "I am fundamentally and irretrievably against it. Ken Livingstone has only ever belonged to one party - the Ken Livingstone party.
"There is no possibility, whatever his immediate future, of the Labour Party being able to rely upon sufficient loyalty and commitment to its aims and obligations."
Prime Minister Tony Blair is thought to want Mr Livingstone back amid fears that Labour's official candidate, Nicky Gavron, could be pushed into fourth place in next year's election.
As well as Mr Kinnock, Chancellor Gordon Brown and Deputy Prime Minister John Prescott are believed to be opposed to the move.
The issue expected to be discussed at a meeting Labour's London regional board on Monday evening.
Labour's National Executive Committee is meeting on Tuesday.
The party's current candidate for the London mayoral elections Nicky Gavron said last week the best option was for her and Ken Livingstone's campaigns to be combined.
Kinnock: Opposes readmission
She said: "During the last few months I have listened intently to the views and concerns of party members and supporters at every level.
"My absolute priority is to reunite the Labour vote in London.
"The best way to achieve this unity is to combine the existing strengths of both our campaigns
and for Ken Livingstone and myself to work together for the good of Labour and