The prime minister's wife, Cherie Blair, has admitted her embarrassment after being fined £10 after failing to buy a train ticket on her way to work as a judge.

Mrs Blair boarded the train at Blackfriars in London on Monday morning without buying a ticket to Luton, where she was sitting as a recorder.

Asked on Monday evening as she left a concert at Goldsmiths' Hall in the City of London, whether it was an embarrassment for her, Mrs Blair smiled broadly and said: "Certainly was."

Earlier, Tony Blair's official spokesman confirmed that Mrs Blair was caught out without sufficient cash for the ticket machine for the £9.60 journey when she boarded the train for Luton.

Mrs Blair, who had just returned from holiday with her husband in Portugal, only had Portuguese currency with her at the time.

She was unable to use her credit card at the ticket machine at the station in London as it only took cash, said Downing Street.

When Mrs Blair arrived at Luton station she told the ticket collector she had not bought the ticket and subsequently paid the £9.70 return fare and the fixed penalty of £10 with her credit card.

'Good grace'

The prime minister's official spokesman said: "She is an upstanding citizen. These things happen.

"She didn't have enough cash. She got on the train mindful of the need to get to Luton on time."

Martin Walter, group marketing director of the Go-Ahead Group, which owns Thameslink, said the ticket machines at Blackfriars were working but do not take credit cards.

He said the ticket windows were open as normal, where Mrs Blair could have paid by card.

Mr Walter said Mrs Blair had taken the penalty charge in "good grace" and he backed the staff member who issued the £10 charge.

He confirmed Mrs Blair offered to pay the full £9.60 fare and £10 fine when she arrived at Luton.

Mr Walter added: "The member of staff issued the penalty charge because that is what they are supposed to do.

"But I understand she approached the kiosk and said she didn't have a valid ticket. It appears that it was just a genuine mistake.

"The employee was just doing his job."