Credit reference agency Equifax has now admitted that 694,000 customers in the UK had their data stolen between May and July this year.
The firm's original estimate of its UK cyber-theft victims, made last month, was fewer, at nearly 400,000.
Equifax now says that it will contact its affected UK customers by letter to offer them help.
It admits they may be at risk of "possible criminal activity".
Nearly 14 million further UK records were stolen, but they contained only names and dates of birth.