A turf-cutting ceremony was held to mark the start of the major overhaul of the George Campbell Centre in March.
The leisure centre will be getting an £850,000 make-over which is expected to be finished in April.
Improvements will include increasing the size of the gym to house 50 pieces of equipment, adding a new exercise room along with new changing rooms and liner for the pool and replacing its roof.
Fenland District Council leader John Clark joined Michelle Tanfield, cabinet member responsible for leisure, at Friday’s ceremony along with Dennis Freeman-Wright, from the Amateur Swimming Association, council staff and contractors GB Construction.
The council has secured a grant for £216,690 of National Lottery funding from Sport England’s Improvement Fund.
Cllr Tanfield said: “I’m really excited that the George Campbell is now getting the kind of revamp that we have all known it has needed for some time.
“It’s a major investment for us which will give people in March and the surrounding area the sort of modern, well-equipped centre they’ve long wanted and deserve.”