The TSB branch, in Hull in the north of England, warned that the monthly interest payments would be a hefty £2.5bn a month - and the money should be repaid in a week.

Liz has admitted that her week long jaunt to the sunshine isle of Gran Canaria - costing around £300 - may have put her a little into the red, but she was stunned by the letter.

"I nearly fainted. I knew I could have been a little overdrawn - but not by £121bn. If I was that much overdrawn I'd have stayed (on holiday) for good."

'I'm sure this is an oversight'

The TSB customer services manager wrote: "I note that your account is overdrawn by that amount although there appears to be no prior arrangement.

"I'm sure this is just an oversight and that you will shortly pay sufficient funds into your account to return it to order and cover cheques you may have issued.

"If it is going to be difficult for you to do this within the next seven days please get in touch with me. We may be able to find a way of resolving the problem."

But the letter warned: "If the overdrawn balance is not repaid within 11 days, your account may be transferred to our Central Collections Department in Brighton.

"In the meantime, interest on the overdrawn balance is being charged at TSB's rate for unauthorised borrowing currently 2.20% per month, but variable.

"Yours sincerely. Manager."

The problem has now been resolved and Ms Seymour's branch have admitted the letter demanding the exact sum of £121,318,928,560 was the result of a clerical error.

Banking gone bonkers

• A London university student was surprised last year when £34,000 appeared in her Barclays Bank account. A week later another £500,000 appeared and by the end of the month she had £1m.
• Northern Rock building society had to apologise to a man from Whitley Bay in the north of England when his bank statement said he owed more then £222m.
• A 14-year-old girl found £15,000 in her account at NatWest in Slough, Berkshire - when she had only just opened it. The money had been deposited by mistake.
• A man took legal action when two building societies credited him with thousands of pounds of other people's money but then his pension company lost £16,000 of his savings.
• A Swedish alcoholic, 55, went on a two-week spree and spent about £55,000 on partying when a bank mistakenly deposited nearly £70,000 in his account.
• In 1992 nearly 500 workers at the Western Isles council were delighted when a computer glitch meant they were all paid twice their monthly salaries.