Santander is warning borrowers about the risks of interest-only mortgages after the sale has completed.
The lender is carrying out spot checks on its interest-only mortgage customers by calling borrowers at random and testing their understanding of the loan they have just taken out.
Borrowers are asked whether they understand they are on an interest-only mortgage, and that they know what this is.
The lender is also checking customers have a repayment strategy and that their broker has explained all the risks.
Interest-only mortgages are seen as risky by some. Last year Citizens Advice said there were 1 million interest-only mortgages in the UK with no clear repayment strategy.
However, the Council of Mortgage Lenders says the risk is reducing,with lenders trying to get at-risk borrowers to remortgage onto different mortgages.
CML research shows that there were around 3.2 million interest-only loans in 2012, when it started collecting data, and that there are 1.7 million now.
The CML says the risk of this volume of interest-only loans “had the potential to be significant”, but that this danger had been headed off.
A Santander spokeswoman says: “We sample a small number of interest-only customers each quarter to ensure they understand the financial commitment of repaying your mortgage.”