It was a top secret military site that was used to create mustard gas bombs more than 60 years ago.But Ministry of Defence officials were back at a former chemical warfare factory on the Norfolk-Suffolk border yesterday as experts began an investigation into potential ground and water contamination.

It was a top secret military site that was used to create mustard gas bombs more than 60 years ago.

But Ministry of Defence officials were back at a former chemical warfare factory on the Norfolk-Suffolk border yesterday as experts began an investigation into potential ground and water contamination.

The Little Heath area of Barnham, near Thetford, was used as a depot to fill mustard gas shells during the second world war and was later used to decommission the deadly weapons and destroy the toxic gas.

Military officials yesterday con-firmed that they had begun a four- to six-week operation at the former military site, which forms part of the East England Military Museum.

A spokesman for the MoD said there was no evidence that there was any threat to public health or the environment, but the organisation was keen to establish the condition of the land. A public footpath that runs through the heath, owned by the Elveden Estate, is set to remain open during the boreholing and testing works.

Officials at the tank museum, which has used the Little Heath site as a tourist attraction since 2005, said they were not expecting the surveyors to find any contamination, 13 years after the site was last decontaminated.

However, trustees had decided to shut the military museum to the public a month earlier than its usual winter closure on health and safety grounds.

Shaun Hindle, trustee of the East England Military Museum, said Little Heath was used by the 94 Maintenance Unit as a Forward Filling Depot for mustard bombs during the second world war.

The deadly chemical was made at Runcorn in Cheshire and was transported by railway to the “top secret” facility at Barnham.

The bombs were never dropped in anger during the war and between 1945 and 1950 the depot, which had three underground chemical storage tanks, was used to decommission the weapons and some of the mustard gas was incinerated on site.

Mr Hindle said the most recent chemical inspection and clean-up was in 1995 when three live mustard gas bombs were discovered and disposed of.

“It is not a problem for us, but in the interests of public safety we have decided to close a month early because they will be digging bore-holes around the site next week. It gives us a chance to tidy up the site over the winter months.

“We have been told that they do not expect to find anything and they have not been called here for any reason other than to do a routine check to make sure that everything is fine.

“It is causing us a little bit of disruption, but we are happy because it will give us another clean bill of health for the site,” he said.

Experts from the Defence Science and Technology Laboratory and consultants from the independent environmental consultancy Entec UK will be sampling soil and groundwater at various locations at the heath.

A MoD spokesman said it was “early days” in the investigation and officials were working closely with the landowners, the Environment Agency, Natural England and St Edmundsbury Borough Council. Barnham Parish Council had also been informed.

“This is a proactive survey and we are undertaking this voluntarily to ensure that it is safe,” she said.

The results are set to be made available for the public record in mid 2009.