Many leaseholders pay excessive ground rent charges of up to £700 a year.
The ministry of housing estimates there were around 4.3m residential leasehold properties in England in 2016.
However, the figure is closer to 6m, taking into account England and Wales there are 6.68m homes, according to campaign group the Leasehold Knowledge Partnership (LKP).
The difference for the gap is because government figures fail to account for many leasehold homes in the social sector, Martin Boyd from LKP told Mortgage Solutions.
In part this is because for many years local authorities did not register the titles of individual flats, just the block.
LKP estimates there are at least 1m flats in the social sector, based on the 2011 census, whereas the government latest estimates put the figure at around 244,000.
In recent years, a number of developers sold houses as leasehold, which the government is set to make illegal next year.
However, Boyd said there is little help for owners of leasehold houses, who are likely to struggle to sell their properties.
Many were first-time buyers – and in the coming months and years – the scale of this issue will be come more apparent, Boyd added.
He said: “I think there will be an awful lot of people getting their fingers burnt.”