Robbers struck two branches of the same building society in the space of half an hour yesterday.

The raids were the latest in a spate of hits on Portman building societies across Sussex, possibly involving the same gang of armed criminals.

Police are linking yesterday's attacks at Portman branches, the first in Lewes at about 3.51pm and the second in Heathfield 30 minutes later.

Three men were seen by staff loitering outside the Lewes branch in the High Street. One held the door open and pretended to be interested in finding out about mortgages before all three charged into the premises.

One vaulted over the counter. Three terrified female staff fled to another room and locked the door.

Police were last night carrying out forensic tests and it was not known what had been stolen, although the men were seen near the tills.

Police said a knife had been spotted on one of the men, who were black, but it was not used.

Malcolm Mortom, Portman practice manager at Lewes, said: " One of them leapt the counter and our staff fled back into the rest of the building, followed by one of the men.

"Staff made sure no one was in the branch and locked the door.

"Our staff are deeply upset and have gone home. Our policy is quite clear - nobody puts themself at risk."

The men were seen running off in the direction of School Hill and along Albion Street.

Half an hour later, three black men carried out another raid at the Portman Building Society in the High Street, Heathfield, using almost identical methods. Again, a knife was seen on one of them.

The robbers escaped by car.

A Sussex Police spokesman said: "Police are linking both incidents and are aware of the other robberies at Portman building societies. Inquiries are continuing."

It was the latest in a series of similar attacks on Portman building societies in Sussex. In February, three men, who were black, stole cash from a branch in George Street, Hove, and there have also been raids at branches in Worthing, Lancing and Haywards Heath in the past few months.

In March, a knifeman vaulted over the counter of a branch in East Street, Shoreham, and made off with cash.