Sir Albert Bore is to be reported to the local government watchdog over allegations that he "bullied and intimidated" the acting chief executive of Birmingham City Council.
The former council leader, presently leader of the Labour opposition group, stands accused of putting pressure on Stephen Hughes to withdraw a witness statement before the High Court.
Mr Hughes, in his role as the council's interim chief executive, said in his statement that there was very little evidence of postal vote fraud at last year's Aston ward by-election.
He did not believe there was any justification in accepting the appeals of five former Labour councillors, who were found by an election court to have organised widespread ballot fraud in June 2004.
Mr Hughes said electoral procedures in Birmingham had been tightened.
He warned that public confidence in the electoral system in Birmingham would be damaged if the High Court ruled the election court to reconsider the case against the five former councillors.
There would also be costs to the council of at least £100,000 in organising fresh election court hearings.
The High Court rejected the appeal, but is still considering a new election petition which alleges ballot fraud by Liberal Democrats at the Aston byelection, where the party won all three seats on July 28 last year.
Sir Albert wrote to Mr Hughes accusing him of using "value judgements" which were prejudicial to the election petitioners.
The letter went on: "You make reference to the petitioners' use of the allegations of massive electoral fraud and contrast these with the low level of complaints to the Elections Office.
"However, you have seen my report to the police . . . so you know that there are many substantiated complaints. I do not therefore understand why you find any level of electoral fraud, low or otherwise, acceptable.
"I am frankly shocked that you do not make any reference in your witness statement to the report that I have submitted to the West Midlands Police Economic Crime Unit. The evidence I have cited in my report goes a long way to doubting the legality of aspects of the election campaign of the Liberal Democrats.
"I therefore take the view that your witness statement comments on the matter of this alleged fraud are at best complacent and at worst partisan."
Sir Albert's comments, particularly those alleging complacency and partisan-ship, will be reported to the Standards Board for England by Aston Lib Dem councillor Ayoub Khan.
Coun Khan said he believed Sir Albert was in clear breach of the council's code of conduct, which forbids councillors from doing anything "which compromises or is likely to compromise" the impartiality of those who work for the council. Councillors must also treat officers with respect.
Sir Albert is standing by his letter. He said Mr Hughes' witness statement was drawn up specifically to deal with the appeal by the five former Labour councillors and ought not to have been used in an attempt to influence the High Court on the new Aston election petition.