You’re heading to the till at the supermarket, your trolley laden. Why not complete the one-stop shop by taking a financial product to the checkout? The answer, says a report by Which?, the consumer watchdog, is that you could lose in money what you gain in convenience.

Borrowing £5,000 from Marks & Spencer (without insurance) over the internet or telephone, with an APR of 9.9 per cent, comes to £800 over the three years. Moneyback Bank’s internet loan charges 5.6 per cent, a more modest £452 over the term.

After Moneyback comes Sainsbury’s telephone loan, with an APR of 6.1 per cent: £305 over three years. This is surprisingly reasonable given how critical the report is of supermarket and “non-traditional finance providers”. However, Sainsbury’s internet loan with an APR of 7.4 per cent, totals £591 over the term — £286 more than its telephone sibling.

Virgin Money counts as another “non-traditional” provider to watch. Its three-year loan of £5,000 has an APR of 6.9 per cent. Andy Britchford, of Moneyfacts, the price-comparison website, says: “Non-traditional providers and especially supermarkets sit in the middle of the road — not the best, nor worst, rates. Indeed, £5,000 loans from Sainsbury’s, Tesco and Virgin are less expensive than those from Abbey and Nationwide.”

Borrowers should be aware that headline rates do not include payment protection insurance. PPI is expensive and not usually worth it, according to a recent report by the Office of Fair Trading.

With PPI, repayments for the Moneyback loan rocket by £38.98 a month — an extra £1,403 over three years.

Mr Britchford says: “Borrowers should look carefully at the terms and conditions of loan insurance to ensure that they could claim it if the need arose. If they really want PPI, buying it from a third-party provider is far cheaper.”

For more on consumer affairs visit www.timesonline.co.uk/consumeraffairs