Masked robbers knocked a security guard to the ground and snatched more than £70,000 as he was carrying the cash into a bank in a market town yesterday.

They struck quickly in the early hours after a Securicor van pulled up on a small darkened access road outside a branch of Barclays in Market Place, Barnard Castle, County Durham.

Detectives believe the thieves may have kept watch on the bank and vehicles for several weeks so they could plan the snatch down to the last minute.

Nothing was seen by police cars which rushed to the area minutes later.

The 23-year-old guard was carrying the money in cassettes which were to be inserted into the bank's cash-point machines. He was badly shaken but was not injured.

Two robbers, who wore dark clothing and balaclava masks, threatened to harm the guard with heavy wooden stakes if he tried to stop them. Another guard in the van raised the alarm.

The robbers then used a getaway route that even many Barnard Castle residents do not know about.

They opened a black wooden door marked "private" near the bank and between the Teesdale Learning Shop and Fine Rug Shop to get into an alley leading to six small flats.

They ran past the flats into the courtyard of a cottage. They crossed that and climbed over a 6ft wall to get into the Hole in the Wall car park. There, a witness saw their hatchback screeching away just after 1.30am.

One police officer said: "They must have researched the area well before choosing that getaway route. But we have no idea which way they drove. They may have taken any of umpteen roads small or large out of the town, and may have switched cars."

A watch was set up on the A66 and other routes but no sign of the robbers was found.

Detective Sergeant Billy Hughes, leading the hunt for the thieves, appealed for anyone who saw strangers acting suspiciously in the hours before the raid, or around the same time in previous weeks, to call police.

Detectives made house to house inquiries yesterday asking people who live in the area if they had seen anyone hanging about lately, or if they heard noises around the raid time.

The bank opened on time yesterday, and business carried on as if nothing had happened.

Linda Fishburn, an area manager for Barclays, said no comment could be made about the robbery, and staff had been instructed to say nothing about it.

* Can you help? Contact Durham Police on 0191-386 4929.