Millions of households that have so far been protected from the credit crunch could be hit with higher mortgage bills after NatWest and Kent Reliance became the first lenders to increase repayments for existing customers.

NatWest, one of the country’s biggest lenders, will raise rates for thousands of customers with a variable rate offset mortgage from 6.2 per cent to 6.45 per cent tomorrow. The increase also affects customers with the same deal from its parent company, Royal Bank of Scotland. On a £150,000 home loan, the rise will cost borrowers an extra £276 a year.

Borrowers with loans linked to Kent Reliance’s standard variable rate, including those on discount deals, have also learnt that their rates are rising. The building society is putting up its standard variable rate by a quarter of a percentage point from 7.34 per cent to 7.59 per cent today.

The increases are the first time this year that mortgage rates have jumped for existing borrowers.

Brokers are worried that more lenders will follow, as the borrowing crisis, which has already resulted in a raft of rate rises for new customers, shows no sign of abating.

Although there are hopes that the official Bank rate could come down as early as next week, the cost of borrowing in the money markets, where banks and building societies raise funds, remains high.

David Hollingworth of broker L&C said: “Given that the Bank rate is expected to fall you wouldn’t normally expect rates for existing borrowers to rise. But in these uncertain market conditions you can’t really count anything out.”

Meanwhile, rates for new borrowers continue to increase. Nationwide, Britain's biggest building society, last week raised rates on its two-year tracker for new borrowers with a 5 per cent deposit from 6.43 per cent to 7 per cent. The building society also lifted two-year fixed rates by 0.2 percentage points.

Cheltenham & Gloucester, the mortgage arm of Lloyds TSB, also raised some two-year trackers by up to 0.35 points, taking its deal to 6.99 per cent.

Bank of Ireland has withdrawn most of its two-year, three-year and five-year fixed rates. It will no longer sell two-year fixed or two-year discounted rates.