Halifax has argued that the cost of funding its mortgages is forcing it to increase the cap on the SVR, but if other lenders come to the same conclusion, millions of people could struggle.
Nearly 2 million people had come off their initial mortgage deals onto cheaper SVRs by 2010, according to the Council of Mortgage Lenders. With Bank Rate at a record low, there has been little incentive for them to find a new deal, so that number is likely to have risen in the past year.
RBS also blamed higher funding costs. A spokesman said: "Over the last year the cost of funds at which we need to borrow to fund our mortgage commitments has risen considerably. We have absorbed the cost during this period but have now decided to pass on some of this increase."
David Black from Defaqto, the financial analyst, said the most financially vulnerable people would be stuck with their lender when rates rose, because their financial circumstances meant they would be unable to remortgage. "Typically those who cannot switch lender are those who have had credit problems, are on very high loan-to-value or interest-only mortgages or those who are approaching retirement," he said. "It will be a nightmare for them. Lots of people are struggling already."
He said that five years ago the SVR on a mortgage was "almost an irrelevance" but now, with so many stuck with their lender and others taking advantage of low mortgage rates, these "revert to" rates should be "scrutinised very carefully".
Mark Harris of mortgage broker SPF Private Clients said there were many people who had received their Halifax mortgages under a "fast-track facility" under which they did not have to prove their income. They may now find that their earnings are not high enough to satisfy the lender in order to go onto other deals.
"These trapped home owners could struggle to afford higher mortgage payments," he said. "Anyone in this position should speak to their lender about a solution, mentioning the concept of 'treating customers fairly'."
New figures from the Bank of England suggest that Britain's home owners have saved a collective half a trillion pounds in mortgage interest payments as a result of record low interest rates. This works out at an average of £50,820 for each of Britain's 11.2 m mortgages. Many of those who have saved on interest are on SVRs.
For many, these savings have been a vital lifeline rather than a perk. "While utility bills and other costs have been rising, low interest rates have allowed people to mop this up," Mr Black said. "If rates rise now, these people will struggle."
Only a handful of SVRs are pegged to Bank Rate. In other cases, mortgage companies are free to do as they wish with the rate. Mr Gregory said the headlines were always grabbed by Bank Rate, and many home owners believed their mortgage would only be affected by this. But in practice banks that want to lend take more notice of Libor, the rate at which banks lend to each other. Halifax said events including the crisis in the eurozone had changed its costs, and other banks may argue the same. "There is lots of uncertainty over SVRs and this brings it out into the open," Mr Gregory added.
Mr Black said the average SVR was now 4.79pc. But some people, notably at Nationwide and Lloyds, are enjoying record low rates that are pegged at 2 points above Bank Rate. These rates are not available for newer borrowers, and both Nationwide and Lloyds operate a two-tier strategy – there is a higher SVR for newer customers.
Mr Harris said that for those customers the record low rate was unlikely to change now. "Those lenders with a 2 point cap above base rate seem to have already explored ways of legally increasing the margin and decided against it, so those borrowers should also be safe."
Other borrowers on SVRs should be thinking very carefully, however, especially if they are able to remortgage. There are a number of good-value fixed and variable mortgages available, while those who are enjoying low rates at present could pay down their loans to increase their equity and make it easier to move.
First Direct, for example, has a tracker mortgage for two years at 1.99pc (1.49 points above Bank Rate), with a maximum loan-to-value of 65pc. The rate reverts to an SVR of 3.69pc; the fee is £1,499. Bank of China offers a lifetime tracker, provided that you have 20pc equity in your home. This loan has a fee of £1,295 and tracks at 2.3 points above Bank Rate, currently 2.8pc. The table has more options.
Those who are struggling to pay their mortgage now and fear they will be unable to do so should their SVR rise should speak to their lender.