Blackburn with Darwen Council's Liberal Democrat group has suspended a councillor who announced he is to switch to another party.
Long serving Coun Paul Browne, the group's former leader, is to stand for the For Darwen party in May's local elections.
The Lib Dems, For Darwen and the Conservatives make up the coalition currently in charge of Blackburn with Darwen council.
Coun Browne had intended to remain a Liberal Democrat until May, but this was dismissed by party leader David Foster as "nonsense."
And in a statement, Coun Foster said the move had come "as both a surprise and a disappointment."
He said: "Coun Browne has not told the group that he was planning to leave the party and in fact denied that he had any plans to do so.
"The party went into the current partnership on the basis of principle and with an agreed programme, not on the basis of personalities.
"The party has not changed its policy either locally or nationally, we are therefore surprised at Paul Browne's decision.
"We have suspended him from membership of the group."
Coun Browne, who is thought to be on holiday in Malta, was unavailable for comment but has previously said he will explain his reasons at next month's full council.
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