The ruling Lib Dem/Tory coalition was at risk of losing its majority on small committees after seven Lib Dems quit to form another group. So Tory leader Roman Scuplak registered as a Lib Dem to preserve the coalition's majority.
He said: 'I am still leader of the Conservative group but for official purposes I am classed as a Liberal Democrat.'
But town clerk Tom Stephenson ruled: 'A councillor may only be a member of one group at a time.'
But leader Roger Blackmore (Lib Dem) said: 'It is nothing of the sort.'
Chief executive Rodney Green was due a performance payment, he said.
Mr Green was on holiday and could not be contacted.