TJX chairman Ben Cammarata said the firm was “extremely disappointed” to discover “an unauthorised intrusion into our computer systems that process and store information related to customer transactions”.

The cut-price clothing giant has not yet been able to rule out impact on UK customers.

“The intrusion could extend to the portion of our network that handles customer transactions for TK Maxx stores in the UK and Ireland,” Mr Cammarata confirmed.

To date, the company has established that the intruder accessed information on transactions conducted during 2003 and from May to December 2006 for stores in the US, Puerto Rico and Canada.

The security breach could fuel fears about the security of banking details following concerns about online banking and warnings from PayPal about the ‘loose lips’ syndrome among UK office workers revealing their bank details over the telephone earlier this month.

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