The consumer champion also found that just a third of people (36%) who are struggling with mortgage repayments have approached their lender for help.
Options available to lenders include moving customers from a repayment to interest-only mortgage, allowing them a payment holiday or allowing them to move to a different deal.
The consumer champion is calling on lenders not to use the current financial crisis as an excuse to neglect struggling borrowers.
Although in over three quarters (78%) of cases borrowers were offered some form of assistance, Which? warns that a significant minority of lenders aren't doing enough to help their customers.
An increase of £50 a month in mortgage repayments would have an impact on three quarters of households. 37% would need to cut back on regular spending, 20% would reduce the amount they are saving and 9% would not have enough for essentials.
Although the Bank of England Base Rate is not expected to rise in the near future, recent research by Which? Money shows that a fifth of lenders have increased the rates on their SVR mortgages since the rate reached its all-time low of 0.5% in March 2009.
Which? executive director Richard Lloyd says:
"With ongoing uncertainty about the economy, it's not surprising that many people are worried about their household finances. For some it would only take a small increase in mortgage rates to send them over the edge.
"If you're struggling with mortgage repayments then your first port of call should be your lender. It's not in anyone's interest for people to default on their mortgage, so it's important that lenders do all they can to help their customers who are struggling with repayments."
