It has long been criticised as, at best, poor value for money. At worst, it has been described as a financial disgrace and a scandal. Yet still we buy payment protection insurance (PPI).

These policies take the form of bolt-on cover for unsecured personal loans, credit cards and mortgages. The idea is that borrowers will be able to maintain their debt repayments in the event of their income disappearing due to illness, accident or unemployment.

An estimated 20 million of these policies are in force today " generating pounds 5bn a year for lenders. But such success has taken place against a backdrop of years of criticism for being overpriced, worthless and riddled with exclusions that can prevent payouts on customers' claims.

And last week, the Citizens Advice consumer organisation declared that this poor deal has gone on for too long. It lodged a 'supercomplaint' with the Office of Fair Trading (OFT) about the mis- selling of PPI policies, dubbing it a 'protection racket'.

Taking evidence from 270 of its bureaux around the country, Citizens Advice says the cover is more a lucrative source of profit for the finance industry than an offer of protection for consumers.

Extra premiums paid for PPI, says the Citizens Advice complaint, can add up to 25 per cent to the cost of the original loan that the insurance policy is taken out to protect.

According to separate research from the price-comparison website moneyexpert.com, PPI can add more than pounds 900 over four years to the cost of a pounds 5,000 loan.

Put PPI into the equation, moneyexpert.com has found, and the companies heading its own 'best buy' lists for loans no longer rank among the top five for value.

The supercomplaint voices the concern that these costly policies can be mis-sold by the lender's staff, often using high-pressure tactics, to 'inappropriate' customers such as the self-employed and contract workers, whose claims would probably not be met by the insurer.

Common ailments that stop people working " a bad back, stress and mental health problems " are often excluded, while many policies prevent claims by imposing arbitrary age limits and excluding pre- existing medi- cal conditions.

Often, buyers aren't aware of all this, either because the policy hasn't been properly explained, or they've missed the details in the small print.