The Guardian (p7) reports that the dispute centres on the government's refusal to agree to free postal delivering for mayoral candidates.
The news comes as Labour's battles over its own candidate nears its climax. Voting for the candidate ends at 5pm on Wednesday with the result expected next weekend.
The government's regulations for the mayoral poll - which fix deposits and spending limits - are due to be debated in the lords tonight, but the crunch is expected next week when they are expected to be defeated unless Labour concedes a freepost mailing for all candidates.
But others, including home office officials and the Electoral Reform Society, believe that may be challengeable in law and that, if the government's regulations are rejected, the election will have to be postponed.