British police have arrested a third suspect in one of the largest robberies in British history after a woman tried to make a large bank deposit.

The 41-year-old woman attempted to deposit the equivalent of $12,000 Cdn in an account at a savings and loans association in southeast London on Thursday.

She was arrested on suspicion of handling stolen goods, a day after thieves disguised as police officers tied up the employees of a security company in southern England and stole as much as $53.3 million Cdn.

A police van arrives at the Securitas security depot in Tonbridge, England, Thursday. (AP Photo/Tom Hevezi)

"If this is anything to do with the robbery, it's absolute stupidity beyond belief," British money-laundering expert Peter Lilley told Reuters. "That's the one thing you don't do."

A 29-year-old man and 31-year-old woman arrested earlier this week were being held at police stations in Kent on suspicion of conspiracy to commit robbery.

• FROM FEB. 23, 2006: 2 arrested after British bank heist

Police have released a computer-generated sketch of a fourth suspect. The man is described as young, white and about six feet tall with a scruffy, ginger beard. He was wearing a police-style cap and a high-visibility jacket, witnesses said.

Police have blamed criminal gangs for the heist at Securitas Cash Management Ltd., and have offered a $4 million Cdn reward for information leading to the recovery of the money.

The heist began when robbers, dressed as police officers, kidnapped the manager of the security company as other fake officers rounded up his family to use as hostages.

One group of robbers took the manager to the security company, where they tied up 14 employees and then loaded the money into a truck. The staff called police about an hour later.