Home loans became even more expensive to repay in October because of higher interest rates and rising house prices, the Council of Mortgage Lenders said.
Many experts are predicting that the housing market will slow
The CML said that interest repayments swallowed up 20.6% of first-time buyers' monthly incomes.
That was up from 20.4% in September and the highest level since 1991.
Meanwhile, Conservative leader David Cameron is asking banks to do more to help householders avoid repossession when their fixed-rate mortgages end.
Mr Cameron wants the industry to take a more sympathetic attitude to borrowers who get into arrears, and to plan now to deal with an expected flood of home owners who might be facing repossession.
The worry is that higher mortgage rates could lead to more repossessions, as a wave of fixed-rate deals come to an end and mortgage lenders boost their rates.
Gary Murphy, a partner in the auctioneers Allsop, says that among the properties they auction, the number of repossessions has risen from 25% to 50% during the past year.
The industry thinks that repossessions will rise by 50%, with 30,000 repossessions by the end of the year.
Michael Coogan, director-general of the CML, said that people in difficulty with their mortgages should contact their lenders without delay.
He told the BBC that it might be possible in some cases to extend their terms, and that no one in the industry wanted to repossess if it could be avoided.
The CML said that although total mortgage lending held up in October, it expected a downturn in the coming months because of the credit crunch.
"October is the last month we expect lending volumes to be higher than a year ago as lenders and borrowers will behave more cautiously in an uncertain and slowing market environment," said the CML's director general Michael Coogan.
"Lenders have already responded to the credit squeeze by tightening lending criteria and increasing some loan costs," he added.
"However, overall, in the coming months we expect the lending figures to be driven more by supply factors rather than lower consumer demand."
Home movers are also being increasingly squeezed.
They saw 17.6% of their incomes consumed by mortgage interest payments, the CML said, up from 17.5% in September and the highest proportion since 1992.
Earlier this month the CML warned that unless interest rates were cut soon to ease the liquidity crisis in the financial markets, then lenders would not be able to fund half their expected mortgage lending next year.
Since then the Bank of England has cut interest rates to 5.5% from 5.75%, with more cuts widely expected.
"For those customers coming to the end of their fixed-rate mortgage in 2008, the potential impact of higher monthly payments will be diminished by the fall in bank rate this month and other rate reductions to come early in the New Year," said Mr Coogan.
The past few months have already seen a sudden slump in the number of new mortgages for house purchases being approved by banks and building societies.
As a result, in October, these loans made up just 39% of all loans secured on residential property, the lowest proportion for nearly three years.
But David Stubbs of the Royal Institution of Chartered Surveyors said: "Significantly, the combination of stagnating house prices and lower mortgage rates in 2008 should boost affordability for the first time since 2001."
"Many would-be first-time buyers, who will have been unable to access the housing market in recent years, will see this as an opportunity to get their first step on the ladder," he added."