Thousands of internet customers are facing large bills after BT neglected to charge them for £30 million of phone calls.

A billing error which went unnoticed for six months led the communications giant to accidentally give many customers free internet access and online time.

Up to a million users of web services managed by BT on behalf of other firms have unwittingly been undercharged.

Now those customers, alleged to include those whose internet access is provided by WH Smith and Tesco, are being hit with large backdated bills.

The problem has emerged at a difficult time for BT - it has been forced into denials of boardroom rows and disputes between its chief executive Sir Peter Bonfield and chairman Sir Iain Vallance.

BT blamed the problem on a "computer glitch".

Calls to some numbers beginning with the 0845 code used to access the internet were misread by BT's computers as free 0800 numbers.

A BT spokesman said: "Obviously we are entitled to claim those call charges, although we will not be charging any extra.

"Customers have been enjoying interest-free credit."

BT refused to disclose the extent of the problem but admitted "tens of thousands" had been affected.

Those whose dial-up access numbers begin with an 0845 code followed by the number six or seven are most likely to receive a backdated bill.

BT has begun to address the problem by correcting the way the numbers are entered into the computer system.

Its next task is to trawl through billing records and send out new invoices.

The average customer is thought to have run up a bill of about £50 during the six-month period which he or she will now be billed for.

The spokesman added: "If a customer gets a higher bill than expected, then we can reach some agreement in terms of paying in installments.

"After all, it was our fault."