Femi Oseni, of Havant, has also been convicted of robbing another branch of the business, the Bradford & Bingley, in Kent.

Maidstone Crown Court was told that 19-year-old Oseni and two accomplices raided the two branches within six weeks of each other in 2006.

The raid on the Portsmouth branch in London Road, North End, lasted just 38 seconds and Oseni and the other two escaped with £220.

They were arrested shortly afterwards at Fratton railway station.

One of the other thieves had the cash hidden in his sock, said Andrew Espley, prosecuting.

Oseni, of Park Road, Havant, Sarah Prince, 27, of Romford, Essex, and Jamie Jones, 18, of Holborn, London, were remanded in custody until sentence on June 13.

As he remanded them, Judge Charles Macdonald QC warned them they were facing 'substantial prison sentences'.

Mr Espley said that during the Portsmouth raid Prince held open a door while Jones, wearing a mask, and Oseni went inside. They had no weapons and made no threats.

The first raid happened six weeks earlier in Sevenoaks where a buzzer system was operated to let in customers.

Mr Espley said a woman appeared at the door wearing a hoodie. She pressed the buzzer and was let in. She held the door open for accomplices who also wore hoods.

Both carried rucksacks. Oseni was armed with a knife and Jones with a spanner. They demanded money from female staff.

Both teenagers jumped over a desk and filled the rucksacks with more than £20,000.

They ran out and joined Prince, who was waiting by a getaway car. The raid took two minutes, said the prosecutor. The robbers were seen by witnesses and the car's registration number was taken. It had been stolen in London.

Police checked CCTV footage and found the car had crossed the Queen Elizabeth Bridge from Essex, and was in Sevenoaks on the day of that robbery.