The losses often become apparent to families only when their loved ones die and they try to sell their home. There are 150,000 retirement flats in the UK. They don’t have full-time nurses but most have communal areas and features to help residents live independently. There is often 24-hour telephone support or wardens on site.

The properties are sold as leaseholds with the freeholds bought by the highest bidder. The freeholder collects an annual ground rent and appoints an agent to run the development. These companies have been accused of levying excessive fees and charges and leaving facilities to fall into disrepair.

Sebastian O’Kelly, of betterretirementhousing.com, said: “These flats routinely plummet in value and the reason is the leasehold system. The freeholder and property manager still get their ground rent and service fees irrespective of price. It’s deplorable that families are pouring money into these purchases, often in desperation, only to see their value evaporate.”

Retirement home builders say the value of the properties is not just financial. They say they reduce loneliness and the burden of maintenance and increase safety and security. McCarthy & Stone points out that since 2010 it has not allowed outside companies to manage its sites and this is protecting values.

Some families have concerns about how properties are sold. One complained that a 88-year-old relative was sold a flat while her daughter was on holiday. When the woman died, the flat wouldn’t sell. Land Registry data shows the average loss of value for flats in the block is £74,000.

The Times looked at nearly 500 retirement flats in 15 developments built between 2001 and 2015. Almost 80 per cent of the homes sold since their first purchase had fallen in value with an average loss of £38,846. The analysis suggests that flats built since 2010 have fared better with only 37 per cent experiencing losses. But one McCarthy & Stone flat built in 2015 lost £45,000 in value when it was sold this year. In the past four years McCarthy & Stone has made profits of £383 million.

Mr O’Kelly said: “The situation may be improving as builders move to being service providers but these companies successfully lobbied government to retain ground rents on retirement sites, which doesn’t encourage the belief they have a long-term interest.”

This week Churchill Retirement Homes donated £150,000 to the Tories. The company is run by Spencer and Clinton McCarthy, the sons of John McCarthy, the co-founder of McCarthy & Stone. There is no suggestion that the donation was linked to the decision to exempt retirement home providers from a ban on ground rents. Spencer and Clinton McCarthy have been Tory supporters for ten years.

The industry says the sale of freeholds funds communal areas and without this system flats would cost more.