The Competition and Markets Authority (CMA) said the "unfair" terms, which double ground every 10 to 15 years, "trap" people.

It said the contract means people can struggle to sell or mortgage homes.

The builders said they had already taken steps that address the issue.

The CMA said it had concerns that the clauses in the contracts may break consumer protection law. They must be removed and not used again, it 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," said CMA chief executive Andrea Coscelli.

"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."

He added: "If these developers do not address our concerns, we will take further action, including through the courts, if necessary."

The watchdog is also looking into Barratt Developments and Persimmon Homes contracts.

The difference between a freeholder and a leaseholder

Someone who owns a property outright, including the land it is built on, is a freeholder.

With a leasehold, the person owns a lease, which gives them the right to use the property. But they still have to get their landlord's permission for any work or changes to their homes.

When a leasehold flat or house is first sold, a lease is granted for a fixed period of time, typically between 99 and 125 years - but sometimes up to 999 years. People may extend their lease or buy the freehold, but this can be complicated and expensive and involve legal fees.

Leasehold house owners are also often charged expensive ground rent, as well as fees if they want to make changes to their homes. A leasehold house can also be difficult to sell.

Campaigners have called for leaseholds to be banned on new builds, and the government has said previously it would work to end the practice.

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."

• Government vows to end complex leasehold costs

Taylor Wimpey said: "We will continue to cooperate with the CMA and work with them to find a satisfactory resolution, within the required timescale."

The housebuilder added that it stopped selling leases that doubled ground rent every ten years on new developments from 1 January 2012.

In 2017 it launched a voluntary help scheme that covers the cost of converting terms so ground rents are linked to rises in the retail price index (RPI) measure of inflation and set aside £130m to cover the cost of lease conversions.

The company said a "significant number" of Taylor Wimpey customers have already used this scheme and it remains open.

Countryside said it had "sold no properties with doubling ground rent clauses since 2017" and that it had an assistance scheme for people who charges doubled more than every 20 years.

It said it would "continue to engage constructively with the CMA to resolve this complex issue."