Targeting homeowners who have no plan to pay off their interest-only mortgage is like a game of Space Invaders, a banking trade body has suggested.
Many thousands of households have no strategy for paying off the lump sum at the end of an interest-only mortgage term, so risk losing their homes.
Lenders have been contacting these people urging them to work out how to save up for this final bill.
Over time this becomes more difficult - like Space Invaders, UK Finance says.
Banks need to improve their targeting in order to engage with those reluctant to discuss how they will pay off their mortgage, according to James Tatch, an analyst at the banking trade body.
"For those of us that remember it, we could think of this challenge as being like Space Invaders: at the start of the game there are plenty of targets, albeit that most are far away," he said.
"It is relatively easy at that point to fire and hit quite a few targets, particularly the nearest ones, even if your aim is not that good. As the round progresses, more and more of the simpler targets are eliminated successfully, leaving the faster-moving, more elusive ones. So, you need to improve your aim, and make your strategy more agile, to pick off the shrinking numbers ahead of you."
Interest-only deals allow borrowers to pay off the amount borrowed only when the mortgage term ends, usually after 25 years.
UK Finance said 1.7 million homeowners had ongoing interest-only mortgages, nearly half the number seen in 2012, when this data was first collected. The total value of the interest-only mortgage book was £250bn, down 37% in the same period, its figures show.
Lenders have been writing to these customers asking how they will pay off the lump sum, but accepted it was "a challenge" getting some to engage. There has been concern among regulators that a host of homeowners do not have plans in place to pay the final bill.
One peak of these final bills has come in the past year or so, for those who took out endowment policies in the 1990s and 2000s.
Less affluent, middle-aged homeowners who often converted to interest-only deals in 2003-09 - who are concentrated in the South West, East and North West of England, as well as London and the West Midlands - will see their final repayment demand come in 2027-28.
Jackie Bennett, Director of Mortgages at UK Finance, said: "There remains plenty more work to do over the coming years to ensure that those remaining borrowers who have so far been reluctant to engage have viable repayment plans in place.
"We continue to encourage all borrowers with interest-only mortgages to contact their lender as soon as possible, as the sooner they do so the more options will be available."
These options include using savings to pay off the lump sum or switching to a repayment mortgage, if possible. Those who are struggling are usually given a period of grace by their lender at the end of their mortgage term to find the money.
In a minority of extreme cases, mortgage customers who cannot pay off the lump sum end up having to sell up or hand over the keys to their lender.
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