The Ministry of Defence has investigated 14 former chemical weapons factories and stores across the UK for contamination, according to an official briefing seen by the Guardian.

Tens of thousands of tonnes of mustard gas, phosgene and other lethal chemicals have been made, stored, burned and dumped at sites in England, Wales and Scotland since the first world war. Some areas are still fenced off to protect the public today.

After a four-year investigation of the sites considered potentially hazardous – named Project Cleansweep – it has concluded that there is "no indication of significant risk to public health or environment" from the sites.

That has been questioned, however, by one expert, who pointed out that there was still no scientific proof that all harmful traces of the weapons have been removed, particularly after they were burnt.

Prof Alastair Hay, an expert on chemical weapons and who is a professor of environmental toxicology at the University of Leeds and an official adviser to the health and safety executive, argued that more sampling might still be needed at the sites. Some areas should be kept secure as a failsafe because mustard gas can be very persistent in the environment, he warned.

"The more problematic areas are where physical destruction took place," he said. "Incineration is a well-recognised method of disposal, but you need to ensure all is burned. We have no details of these procedures."

Chemical weapons were extensively used by both sides in the first world war, and have been blamed for 100,000 deaths.

Soldiers particularly feared chemical gas attacks because they could cause blindness, blistering and a slow, painful death. The weapons are now outlawed by international agreement in most countries.

The MoD launched Project Cleansweep in 2007 to provide "reassurance" that residual contamination at UK sites did not pose a risk to human health or the environment. A briefing on the project has been released in response to requests under freedom of information law.

The project initially considered 46 sites, but whittled that down to 14 that required detailed investigation. Although sites may have been cleared in the past, "we do not have scientific evidence that all harmful traces of the agents were removed or disposed of", the MoD briefing stated.

The 14 sites included two former US chemical weapons stores at Aberfoyle in South Lanarkshire and at Worksop in Nottinghamshire. At Bowes Moor in Durham, 17,000 tonnes of chemicals were stored on 564 acres of moorland.

According to the MoD, areas historically used for the disposal of mustard gas are still fenced off at Riseley in Bedfordshire and at Spalford Warren in Nottinghamshire. Spalford Warren has also been designated as a site of special scientific interest because of "its importance as a grass-heath habitat".

The sites are now safe, the MoD briefing said, and "suitable for their current use, provided any management systems, restrictions or procedures remain in place". A comprehensive report on Project Cleansweep is due to be published later this year.