The former GLC leader's supporters reacted angrily after it was revealed that the panel formed to draw up a shortlist of candidates was heavily skewed against him.
In the most serious setback to date for Mr Livingstone's bid to become Britain's first directly-elected mayor, The Independent has learnt that the 12-strong panel is dominated by staunch Millbank loyalists.
The membership of the body emerged as a new controversy erupted over the Brent East MP's comments that government cuts were "whacking" the poor in the capital. Writing in the London Evening Standard, Mr Livingstone claimed that the most vulnerable had been hit hardest by a recent round of council cuts in London.
The comments contrasted with an earlier open letter to Tony Blair in which the MP said that he would not use the mayoralty to wage political warfare against the Government.
According to a confidential Labour document approved by the party's national executive committee, the vetting panel for all Greater London Authority candidates will comprise four NEC members, four London party members and four "independent" members.
The NEC figures are the Parliamentary Labour Party chairman, Clive Soley, with two loyal officials from the MSF and AEEU unions.
The Greater London Labour Party members are dominated by the "stop Ken" camp, as is the independent group. Worse still for Mr Livingstone, the document states that the "disciplinary and voting record" of all candidates will scrutinised.
The detailed selection procedures for the mayoralty are to be decided over the next two months, but it is understood that the panel will be used for both GLA assembly and the mayoral contests.
Geoff Martin, London convener of Unison and a former member of the London Labour Party Board, said the moves proved that Millbank officials were determined to block Mr Livingstone. "It is just so transparent now that this is an attempt to kneecap Ken and the ordinary Labour voters and members will see it as such. Choosing a sanitised list of panel members is another attempt to erect barriers to his candidacy. The people who should really have a say are the ordinary members."
But Jim Fitzpatrick, MP for Poplar and Canning Town and chairman of the London Labour Party, said Mr Livingstone's conflicting statements proved he was trying to "dupe" party members. "It seems Ken has changed his mind about being loyal to the Labour Party before the ink is dry on his letter to Tony Blair," he said.
"If Ken cannot keep his word for a mere 24 hours, how can anyone believe he would stick to his promises over a four- year mayoral term?"
Joan Ryan, MP for Enfield North and a member of the London Party Board, said the selection panel was representative of the ordinary party members in the capital.
"Ken's views and those of his supporters are marginal within the Labour Party, so there is no reason why they should be on a selection panel," she said.