The scale of the error, in which tens of thousands of homes in the South- east and South-west of England were sent false statements, was only spotted yesterday.
Abbey initially claimed only 100 or so borrowers, some of whom received statements telling them they were liable for payments up to 50 per cent larger than before, had been affected by the mistake.
One Abbey National borrower, who declined to be named, said: "My letter said I would have to pay pounds 230 a month instead of the pounds 150 I was expecting. It was not so bad for me, but if I had been an old lady getting my statement, I might have had a heart attack."
A bank spokeswoman said the problem appeared to have happened at one of the computer and printing centres to which Abbey sub-contracts its work. The centre was sent spools with details of the mortgage rate reductions that are being implemented for all itsborrowers from 1 September.
"The spool with the information broke and a number of wrong letters were sent out," the spokeswoman said. "Initially, it was thought that only 100 or so customers might have been affected."
Although Abbey National now believes 40,000 customers may have been affected, fresh letters will be sent out to all borrowers on that print run.
This latest computer problem follows complaints from National & Provincial borrowers, the society recently taken over by Abbey, that thousands of direct debit payments had been inadvertently cancelled.
Abbey has also admitted that thousands of N&P mortgage applications could be delayed because of its merging of 22 administration centres into one site in Bradford. Former N&P members have also alleged irregularities over the non-appearance of cash or cheques they may be entitled to following the takeover by Abbey.