The Competition and Markets Authority (CMA) says it has written to the firms outlining specific concerns over their use of terms that double ground rent every 10 to 15 years.
Campaigners have called for leaseholds to be banned on new builds, and the Government has said previously it would work to end the practice, which has been described as the housebuilders’ equivalent of the PPI mis-selling scandal.
Investigations into Barratt Developments and Persimmon Homes remain ongoing, the watchdog added.
The CMA said that as a result of the increases in ground rent, terms built into contracts, it means people can struggle to sell or mortgage their homes, and risk becoming trapped.
The watchdog is demanding the removal of ground rent terms which officials think are unfair from all existing Countryside and Taylor Wimpey contracts to make sure they are no longer in breach of the law.
Bosses must also agree not to use the terms again in any future leasehold contracts, or risk future sanctions.
Andrea Coscelli, CMA chief executive, said: “These ground rent terms can make it impossible for people to sell or get a mortgage on their homes, meaning they find themselves trapped. This is unacceptable.
“Countryside and Taylor Wimpey must entirely remove all these terms from existing contracts to make sure that they are on the right side of the law.
“If these developers do not address our concerns, we will take further action, including through the courts, if necessary.”
Housing Secretary Robert Jenrick said unfair practices, including crippling ground rents, have “no place in our housing market”.
He added: “This behaviour must end and I look forward to appropriate redress being forthcoming for leaseholders.
“The Government is pursuing the most significant reforms to leasehold in forty years, including by protecting future homeowners, restricting ground rents in new leases to zero and ending the use of leasehold in new houses altogether.”
Countryside and Taylor Wimpey now have the opportunity to respond to the CMA’s detailed concerns and avoid court action by signing formal commitments to remove the ground rent terms from their leasehold contracts.
The investigation will continue, including into investment firms which bought freeholds from developers and continue using the contracts.