The consumer watchdog discovered that 70% of mortgage-holders were concerned about an increase in interest rates with 14% saying they were already struggling with repayments.
Which? said the greatest impact of the latest rises will be felt by “mortgage prisoners” who are unable to move to another provider.
Three quarters of mortgage holders said they would be affected if their repayments increased by £50 a month with 41% saying they would need to cut back on regular spending.
A fifth would need to reduce savings and 11% would not have enough for essentials.
An increase of £100 a month would see a fifth of mortgage holders not having enough for daily essentials like food and 11% being unable to pay their mortgage.
Consumers also said the emotional impact of increases in mortgage repayments would be “devastating” or “a disaster”.
Peter Vicary-Smith, chief executive of Which? said: “Our advice to anyone struggling with their mortgage repayments is speak to your lender straight away. It is encouraging that a third of people we spoke to had approached their lender but worryingly in one in five cases, they said their lenders offered no help at all.
“This is just not good enough and we want to see banks do more to help their customers who are struggling.
“These SVR rises are the consequence of the lack of competition in the market and the failure of the government to take action to promote competition. This is why the new financial regulator, the Financial Conduct Authority, needs to be a watchdog not a lapdog. It must stand up for consumers and stand up to the banks."
