The Greens have become the largest political group on Lancaster City Council for the first time in history.
The change in the make-up of Lancaster City Council came after Heysham South councillor Phil Bradley officially left the Labour group over the weekend.
Councillor Bradley is now a non-aligned independent on Lancaster City Council.
His departure from the group means the number of Labour councillors has dropped to 21. The Greens have 22.
Labour had 23 councillors until recently.
Another Labour councillor, Sophie Maddocks, has resigned from the city council. She previously represented Scotforth East, having been elected last year.
A by-election will be held in due course.
A spokesperson for the Labour group said: "The entire Labour Group would like to put on record their heartfelt thanks to Sophie Maddocks who has recently resigned from her councillor post for personal reasons.
"Sophie put her heart and soul into her work as councillor for Scotforth East and as a kind and energetic colleague to us all.
"We wish her well and hope that she will be able to return to the Lancaster Council Labour team again, bringing her considerable knowledge and empathy, at some point in the future.
"The structure of the Cabinet and the alliance administration agreement are unaffected by these changes. We know our residents just want us to get on with the job and that is our focus."
There are seven Liberal Democrats, five Conservatives, three Morecambe Bay Independents and two independents on Lancaster City Council.
Gina Dowding, deputy group leader of the Greens, said: "We're delighted that the Greens are for the first time ever, the largest group on Lancaster City Council.
"It's an indication of the support we've got not just in Lancaster, but increasingly in the rural areas.
"After the by-election in Scotforth East, we will look to have discussions with our coalition partners about how we go forward."
Councillor Phillip Black, of Labour, is the council leader having been elected in 2023 for a period of two years.
There is no party in overall control of the council, whose cabinet is made up of a coalition of Labour, Green and Liberal Democrat councillors.
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