A controversial prison ship currently moored off the Dorset coast could be moved to London to tackle its spiralling inmate population.
400 inmates are currently held at HMP Weare, off the Dorset coast
The Metropolitan Police is in talks to move HMP Weare to the Thames from its current berth at Portland.
Last month, the ship was condemned as "merely an expensive container - and in the wrong place" by a prison chief.
But the Metropolitan Police Authority, which may have to finance the move, said it "would not go ahead".
In a statement the authority said the idea was considered but was "not practical".
A Metropolitan police spokesman said, despite the authority's comments, the force was in talks.
"The Metropolitan Police Service needs to increase its cell capacity and is currently in negotiations with the HM
Prison Service in relation to the use of a prison ship," he said.
"Other options currently being considered include mobile custody vehicles, using current police premises differently and opening closed custody suites.
"This will be part of a long-term strategy."
A Home Office spokeswoman said no decision had yet been taken on the ship's future but if it were sold a "competitive price" would be sought.
Chief Inspector of Prisons Anne Owers said it should be shut down, unless a massive amount was spent on refurbishment.
A decision on HMP Weare's fate is due at the end of the year.
HMP Weare was originally a troop ship in the Falklands war and then a floating jail in the US.
The government purchased it in 1997 as a temporary overcrowding measure and intended to close it in 2000. It now holds 400 inmates.
Although the jail was generally a safe place, last month inspectors said it was "unacceptably cramped and claustrophobic" with no access to fresh air in
Ms Owers said: "Despite the best efforts of staff or managers, HMP Weare is entirely unsuitable for its present function as a 21st-century category C