A borough solicitor's report has found Keith Fitchett, deputy leader of the Liberal Democrat group, breached the national code of conduct by discussing with council officers a planning application which affected his own home.
Labour group leader Jim Dickson said Mr Fitchett's position was untenable and he should resign. Mr Fitchett has since apologised.
But Liberal Democrat leader Mike Tuffrey said he was astounded at the attempt to make political capital out of the issue, given that Labour member Charles Fairbank had been asked to resign over dual council tenancies).
He was confident he would be exonerated by the inquiry, but the group had a 'one strike and you're out' policy for members, he said.