Solidarity, the party set up by Tommy Sheridan after his departure form the Scottish Socialist Party, has lost its only elected representative in Scotland.
Ruth Black, 43, who won a seat on Glasgow City Council in May, the party's only victory as it fought council and Scottish Parliament seats across the country, has defected to Labour.
She had been a member of the SSP and then Solidarity for the past eight years.
After talks with Glasgow City Council leader Steven Purcell, she decided to join the Labour group. Ms Black said she was approached by Mr Purcell about joining.
She said: "I am very excited to be joining the Labour group at Glasgow City Council and I am looking forward to working alongside my new colleagues.
"Being part of the administration will give me an opportunity to assist with the equalities agenda. This is a move I have been considering for several weeks.
"It is not a decision I have taken lightly. I am certain that it is the right one and I hope I can make a contribution to the work of Glasgow City Council."
Steven Purcell said: "I am delighted that Councillor Black has chosen to join the Labour group.
"Her experience in the area of equalities will be invaluable to our administration and I am certain she will make a valuable contribution to our work in this area. "
A spokesman for Solidarity said last night: "We have only just found out and we are disappointed."
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