Police investigating Britain's biggest-ever cash raid at a security depot in Tonbridge today released the first image of one of the armed robbers.

Two e-fits of the man who posed as a police officer to trick the manager of the depot were released after detectives announced they had arrested a third person in connection with the robbery.

Kent police said a 41-year-old woman had been detained on suspicion of handling stolen goods at a building society in Bromley, south-east London, yesterday.

The woman had been trying to deposit bundles of cash bearing Tonbridge markings at the Portman Building Society when staff alerted police.

A 29-year-old man and a 31-year-old woman were detained yesterday at two separate addresses in Forest Hill, south-east London, on suspicion of conspiracy to commit robbery.

Kent's assistant chief constable, Adrian Leppard, said the manager of the Tonbridge depot and his family had been left traumatised by the events.

Colin Dixon, his wife Lynn and their son Craig were kidnapped during the raid. Today is Craig's ninth birthday.

Speaking at a press conference at Kent police headquarters in Maidstone, Mr Leppard pledged to protect witnesses who may be able to offer information about the robbery.

He urged those "around the edge" of the crime who knew who was responsible to come forward. He said: "You may be fearful about ringing in but I urge you to do so. We will look after you."

He also urged members of the public to report any suspicious activities. "We have callous professional gangsters that we need to catch," he said. "We are going to bring these people to book and we need your help to do so."

Police announced the discovery last night of the Volvo saloon car they believe was used by the gang to abduct Mr Dixon.

The car, which had been set on fire, was found in the village of Leeds, near Maidstone, around midnight. Officers were alerted by firefighters called to deal with the burning vehicle.

Mr Leppard said this was significant as it meant members of the gang were still in the local area last night.

Mr Dixon's silver Nissan Almera, which had been abandoned outside the Cock Horse Inn in Detling, near Maidstone, when he was abducted, was also recovered.

Scores of crime officers are carrying out forensic examinations of the two vehicles and a van thought to have also been used in the abduction of Mr Dixon and his family.

The van, believed to be a former Parcel Force vehicle, was found abandoned at the Hook and Hatchet pub in Hucking, also near Maidstone.

Police said the most important vehicle, the white Renault lorry used to take the cash from the depot, was still missing. It was captured on CCTV.

The armed gang, consisting of at least six people, stole up to £50m in new and used notes in the Wednesday morning raid on the Tonbridge security depot.

The robbery began when members of the gang dressed as police officers flagged down Mr Dixon as he drove home on Tuesday night.

He was taken to a rural location and then driven to the depot in a dark vehicle.

Other gang members also posing as police officers kidnapped his wife and son from their home in Herne Bay, threatening to kill them unless he cooperated.

They were driven to somewhere near Faversham, Kent, before being put in the back of a red van and taken to a building in a rural area.

From there they were put in the back of a white van and taken to the depot where 14 other members of staff were bound and held at gunpoint while the gang loaded the cash into a truck.

A police expert worked with the manager of the depot to compile the e-fit of the gang member who wore a police-style hat and high visibility jacket.

He is described as white, around 6ft tall, with a scruffy ginger beard that could have been false because it was a different colour from his hair. He had freckles on his cheeks and dark eyebrows.

Securitas and its insurers yesterday offered a £2m reward for information leading to the capture of the gang - one of the biggest rewards in criminal history.

Detectives hope it will persuade those on the periphery of the robbery, which is believed to have involved a large number of people, to come forward.

The 100-strong Kent police team is working with Scotland Yard, the National Crime Squad and the National Criminal Intelligence Service to try to catch the gang before they leave the country.

Officers have already received more than 400 calls from members of the public, which are being followed up by detectives.

Police have not confirmed exactly how much money was stolen, and examinations of the depot by forensic experts have delayed attempts by Securitas to establish the amount.

Anyone with information should contact the Kent police incident room on 01622 652361 or 01622 652366.