Hillingdon Council is taking legal action to reclaim land that it argues previously formed part of the public park, and was illegally annexed by a property developer in 2010.
Four houses were built on the site next to Hillingdon Court Park in Uxbridge.
The council said it later discovered that the gardens of the four houses had been extended so that they encroached into the public park.
According to Hillingdon, aerial photography, council records and measurements taken at the site, support its claim.
It said it had entered into discussions in an attempt to resolve the matter amicably, but it now had to begin legal steps to reclaim the land.
Cllr Ray Puddifoot, Leader of Hillingdon Council, said: "Encroachment onto public land is an ongoing problem for many local authorities. We will always take action when public land that is rightfully there for the enjoyment and benefit of our local residents, is unlawfully taken away by private developers.
"Anyone found encroaching onto public land will be asked to reinstate the correct boundary. Those who fail to do so may be prosecuted."