Cllr Danny McCarthy has accused Southwark's Lib Dem group of subjecting him to a "kangaroo court" as they suspended him for failing to pledge loyalty to the party.
The suspension follows weeks of speculation about the possible defection of one or more Liberal Democrat councillors to the Labour Group and rumours of discontent with the leadership of Nick Stanton.
"As many people are aware, Danny has made a number of statements over a period of time which have given cause for concern," said Liberal Democrat Group chair Cllr James Gurling in a statement issued on Tuesday.
"Danny has twice been asked to clarify the situation but – despite making positive sounds – has not yet felt able to put the issue beyond doubt by providing the clear statement of commitment to Group loyalty and responsibility required of him by Group Officers."
"Danny has asked for more time to consider his future and we have been happy to allow him the space he feels that he needs. After a careful and considered discussion last night the members of the Group voted unanimously in favour of suspending him from membership of the Group with immediate effect. This suspension will be lifted as soon as he provides a statement of commitment to the Liberal Democrat group."
"Danny has been a stalwart of the Group for many years. He has twice been elected as a Liberal Democrat – most recently only 6 months ago. I very much hope that he will conclude that his place remains within the Liberal Democrat Group, not least so that he can continue to play his part in the Administration's programme of regenerating the borough, bring power down to local people through Community Councils, building schools for the future, providing 21st century leisure facilities, doubling recycling rates and keeping council tax down."
Cllr Danny McCarthy has represented Cathedrals ward since 2002 and is the chair of Borough and Bankside Community Council. He previously served as a Labour councillor elsewhere in the borough and was Mayor of Southwark twice in the early 1990s.
He confirmed that he is reviewing his political future "because of the Lib Dem/Tory alliance".
"I have I believe been subjected to what I can only describe as a kangaroo court and to be suspended from group, to have been denied natural justice and denied my human rights left me no alternative but to make a final decision on my political future, which I will do by 30 November."
He went on to describe the Lib Dem Group's handling of the matter as "an affront to democracy".
He added: "I bear no ill will against any member of the Group for the decision that was reached, but I would ask them to search their conscience and ask
themselves – Do they think that what was done was fair?"