Homeowners are hit by an extra £2,445 on average on their mortgages by failing to switch to a better deal when their introductory offers end.
The higher payments kick in as 33% of homeowners are moved onto their lender's standard variable rate (SVR) where they stay for an average of 21 months, according to research from uSwitch.com.
Trying to meet the high payments forced 9.7% of homeowners in to debt while 29% said they did not realise their initial deal had come to an end and 38% said they relied on lenders to notify them when period is ending.
The average SVR across the UK mortgage market is 4.49% - 2.83% higher than the average initial discounted variable rate deal.
Tashema Jackson, money expert at the comparison site, said that lenders rely on "borrower apathy" by enticing them in with a competitive introductory rate and counting on them to stay put once the deal is over.
Of the 2,004 mortgaged UK homeowners who were questioned in an online survey, 44% said they were not sure or had no idea at all when their current deal ran out. There were 40% who said their mortgage provider let them know in advance that their initial deal period was about to expire.
Despite having been through the process before 60% of all homeowners said that finding a new mortgage deal was stressful, 36% described the process was time-consuming and 24% find securing a new deal confusing.
Ms Jackson said: "At the very least, borrowers should be able to bank on their lenders notifying them ahead of time when their introductory period ends.
"Homeowners can also make the most of great mortgage deals on the market by planning ahead. If they know when their initial deal is due to expire, they can often avoid higher repayments by shopping around and comparing fixed or discounted rate products."