The Mayor of London has offered to take control of Hampstead Heath in order to save its historic ponds.
The ponds have been used by bathers since the 1860s
In a letter to the Corporation of London, Ken Livingstone said there was no reason why he should not be in charge of the "key London asset".
Every year, an estimated 200,000 people swim in the north London ponds which have been used since the 1860s.
The corporation, which is said to be struggling to cover rising costs of the ponds, dismissed the Mayor's offer.
A spokesman for the corporation, which took over management of the heath in 1989, said: "It was entrusted to the corporation by Parliament 15 years ago and we fully intend to be managing it in 150 years or even 1,500 years from now.
"I must stress that we are still listening to users about a solution and our minds are not at all closed on this matter."
But Mr Livingstone said: "If the Corporation seriously cannot find the resources to fulfil their duty to maintain these facilities on Hampstead Heath, I would be happy for them to be transferred back to London government.
"I will be writing to the City of London Corporation today to ask the corporation to keep these historic ponds open and, if they feel unable to do so, will seek the transfer of Hampstead Heath to my office."
The corporation said it has invested £50m in the heath and was committed to preserving the leisure facilities. A meeting with swimmers was due to take place this week.