Thousands of UK homeowners could face crippling increases in mortgage payments of up to 50 per cent, as banks and building societies move to re-classify them as bad risks, experts have warned.

The warning from mortgage broker Moneygate follows the sub-prime lending crisis in the US, which is prompting banks and building societies in the UK to look at tightening their lending rules - hitting anyone with a slightly patchy credit history.

"These mortgage customers currently enjoying fixed-rate deals could find themselves labelled 'sub-prime' when they come to re-mortgage, getting hit with rates of up to 2.5 per cent above the market rate in the process," Moneygate said in a press release.

Financial experts at Moneygate fear this could spell financial ruin for many householders and lead to a rush of repossessions.

Fixed-rate mortgage customers with even a small blemish on their financial record should look to the market now before it's too late, according to Moneygate.

Sub-prime lenders offer loans to people with a poor credit history. Sub-prime borrowers are much more susceptible to interest rate rises and the slowing of house price appreciation.

Dennis Reed, director of Moneygate, said: "The mortgage market is changing by the day. As lenders look to tighten their terms a person could be labelled a bad credit risk and sub-prime just because of a small financial error in their past.

"The knock-on effect of that re-classification is very significant - a mainstream mortgage payer being shunted into the sub-prime market could face crippling interest charges of up to 2.5 per cent higher than average."

He added: "People applying for mortgages will also need to be much more accurate about the information they give. For example, a county court judgement that in the past was not considered crucial, could now mean the difference of being reclassified as sub-prime when they come to re-mortgage.

"A lot of people are in for a shock and they should think about changing their mortgages as soon as possible before the market does.

"A skilled broker should be able to hunt around now for the best deals available and anyone with a mortgage due for renewal in the next six months should really check their situation, before it’s too late."