Nine out of ten of the UK's mortgages are based on variable rates, making the borrowers extremely vulnerable to rising interest rates, leading investment group Legal & General estimates.The Bank of England held UK interest rates at 0.5% today, making it two years since they last changed, but any rises could have serious consequences for the housing market, L&G says.It estimates only that 1.2m of a total of the UK's 11.7m mortgages are fixed-rate deals, with the remainder based on variable interest rates that will go up if base rates rise, which it forecasts will happen twice this year, once in May and once in the summer.Millions of homeowners rolled on to their lenders' standard variable rate (SVR) at the end of their fixed-rate deal since March 2009, L&G believes, a completely different viewpoint from the Financial Services Authority's (FSA) official data, which record a much high figure for fixed-rate deals.Official data suggests 68% of households are on variable rates, though L&G says the FSA does not include those who have just switched straight on to a variable rate without remortgaging."If banks pass on any rate rises, which seems likely given pressure to rebuild capital levels, many homeowners will quickly experience a meaningful impact on their cashflows," Tim Drayson, economist at the fund manager said.In the previous decade, fixed rates mortgages never accounted for less than 20% of the total market, he added.