Mystery this afternoon surrounded the cause of a smell that alarmed Dereham householders and led to the evacuation on busy market day of a number of banks.

Shoppers and office workers started complaining of a strong gas-like smell in the town from mid-morning.

And National Grid started fielding a stream of calls from anxious families fearing a leak in their homes, especially as lunchtime neared.

The HSBC, Norwich & Peterborough and Halifax bank branches in the town centre closed for business and were evacuated.

However, traffic still flowed as normal through High Street and market stallholders continued to serve their customers.

A handwritten sign asking people not to use the cash machine was put up at the HSBC bank, and another sign on the same building read 'Closed due to potential gas leak'.

National Grid, which handles the nation's gas emergencies, drafted a team of six workers plus a supervisor into Dereham as calls from the public about the smell mounted up.

But, as they continued their investigations for the source, spokesman Raymond Miller said they could find no evidence of a gas escape.

'People have been ringing us up and we are going to their homes. We are literally going from one property to another, but nothing is coming up,' he added.

'Thus far we have no readings of any gas escapes in the area.'

Mr Miller said that by 1pm National Grid had received more than 30 calls from people in the Dereham area reporting the smell.

He said the team would continue to investigate, though the suspicion was that some form of chemical was to blame for the smell rather than domestic gas.

Daniel Sizeland, 28, who runs a market-day fruit and veg stall in Dereham, said: 'I was standing at the stall between 10am and 10.30am when there was a large whiff of gas. It reeked.'

The HSBC branch had reopened by 1.15pm, as had the Halifax bank.

Norfolk police, who maintained a presence during the alert, confirmed that no gas leak had been found. And a member of the National Grid investigating team was calling at shops and offices in High Street later to give the all-clear,