The sacking by Labour of almost 20 Glasgow councillors will see around half the administration stand down before next May's election.

Senior figures in Glasgow City Council's Labour group have told of colleagues being visibly upset as more have found out the party has effectively ended their political careers.

Currently, around 17 of 47 Labour councillors have been told they have not been selected as potential candidates. With two suspended, several ill and a number retiring, up to 25 of the current crop look set for the off.

The names now emerging of Labour stalwarts affected ahead of the 2012 election include James McNally, the former head of the licensing board, city treasurer and executive member for roads and parks; licensing board chairman Stephen Dornan; and chairman of council-owned firm City Building Jim Todd.

The Labour group meets today – with rumours of a challenge to council leader Gordon Matheson seemingly ill-founded. The cull has been driven by London.

One party source said: "The guy from London, Ken Clark, has done this across the London boroughs and knows all the backgrounds of those being interviewed.

"If he's in your interview, it's bad. They've been savage. I've never known anything as brutal."

The move comes as Labour faces the biggest challenge to its grip on the council in more than 30 years after the SNP's buoyant performance in May's national election.

Names emerging of potential replacements, include former MSPs Frank McAveety and Bill Butler, ex-councillor Chris Kelly, James Adams, an RNIB campaigns manager who works with MP Anas Sarwar, Soryia Siddique, a lecturer at Cardonald College, and Aileen McKenzie, another Sarwar ally.

A Labour spokesman said: "Competition is tougher than in previous years."