Allen & Overy is holding one of its partners joining Skadden Arps Slate Meagher & Flom to a nine-month notice period.

In a highly unusual move, London partner Simon Toms will remain at the Magic Circle firm on gardening leave until June next year, despite the fact he resigned for Skadden alongside fellow corporate partner George Knighton in September, according to two people close to the process. Knighton has already started at Skadden.

Although it is common for partners to be subject to long notice periods, it is rare for firms to enforce them, according to lawyers and recruiters.

"In about a third of cases, partners are held to any length of notice, usually two to three months, but more often than not they get to go," commented one recruiter. "Once someone is going, it is expensive and disruptive [to keep them]," he said, adding that holding someone to a long notice period can be perceived as a "sign of vulnerability".

One person with knowledge of the situation said: "It seems like they are trying to punish Simon."

Toms is still listed as a partner on A&O's website but the link to his profile is broken.

He is a regular adviser to 21st Century Fox and was part of a team that advised on its £24.5 billion bid for Sky last year. Other clients he has worked with include Virgin Active, News UK, the Rwanda Development Board and artificial intelligence startup Deepmind.

Another person said Toms and Knighton had been in favour of A&O merging with O'Melveny & Myers. Long-running talks between the two firms collapsed in early September. A separate person said the departures and notice period were unrelated to the merger talks.

Knighton and Toms did not respond to requests for comment.

A&O declined to comment.

The way firms treat departing partners has come into focus following Cleary Gottlieb Steen & Hamilton's decision to fire a four-partner team last week after the firm learned of their planned exit to Freshfields Bruckhaus Deringer.

The move raised eyebrows among rivals, with some saying it was a case of "sour grapes". One former Cleary partner added it was a "very un-Cleary" move to make.