The Ministry of Defence has come under fierce fire for removing radiation warning signs from the nuclear convoys that regularly trundle Scotland’s roads.

The well-known trefoil symbol indicating hazardous radioactivity is no longer used on lorries transporting plutonium or highly enriched uranium for bombs. According to the MoD, this is so it can maintain its policy “to neither confirm nor deny the presence of nuclear weapons.”

The move has been fiercely criticised by SNP and Labour politicians for endangering the public and emergency services. Campaigners say the MoD is putting secrecy ahead of public safety.

In the past vehicles carrying “special nuclear materials” such as plutonium and uranium displayed the warning signs, though those carrying nuclear weapons did not. But following a decision to use the same vehicles for both kinds of shipments, the MoD dispensed with all radiation symbols.

The change was only revealed in a recent parliamentary answer from the defence minister Penny Mordaunt to the SNP MP for Midlothian, Owen Thompson. “This beggars belief,” he told the Sunday Herald.

“If the UK government really thinks it is better for people not to know nuclear material is being driven on motorways and through city centres to feed their nuclear obsession, it is simply untenable.”

He accused the MoD of trying to hide its “deadly” loads. “To cynically pull the wool over people's eyes about this awful practice of nuclear convoys through our towns and cities by not even owning up to what is on the trucks is unacceptable.”

Bill Butler, a Glasgow Labour councillor and convener of the nuclear-free local authorities in Scotland, described the MoD’s decision as “ridiculous”. He pointed out that local councils weren’t informed about convoy movements.

“Now their staff, and other emergency service staff, may be putting themselves at risk or even harm by being unaware of the serious radioactive content of a convoy in the event of a serious accident,” he said.

“And all for a change in defence doctrine which will not acknowledge there are nuclear weapons being transported through Scotland, when quite obviously they are.”

Road convoys between the nuclear bomb factory at Burghfield in Berkshire and the naval weapons depot at Coulport on Loch Long are frequently monitored by anti-nuclear campaigners. Most recently, one was photographed and tweeted about on Scottish roads on 15 February.

Jane Tallents from the monitoring group, Nukewatch, accused the MoD of refusing to tell fire and ambulance services about convoy movements. “If one of these convoys is involved in an accident, would firefighters arriving first on the scene have to wait until police turned up to find out that they were dealing with a highly hazardous radioactive cargo?” she asked.

“The MoD is putting secrecy about its nuclear weapons before the safety of the general public, who it is supposed to be protecting. That can never be right.”

John Ainslie, coordinator of the Scottish Campaign for Nuclear Disarmament, accused the MoD of taking a “cavalier attitude” to nuclear safety. “The MoD should not be allowed to move large amounts of plutonium in unmarked vehicles,” he said.

The MoD confirmed that vehicles carrying special nuclear materials have not carried warning signs since 2012. The change “was needed in order to maintain the policy to neither confirm nor deny the presence of nuclear weapons,” it said.

“The safety of the public is always our priority and convoys are conducted to the strictest safety standards, which are approved by the independent safety regulator,” said an MoD spokeswoman.

“The MoD has well-established and robust arrangements in place with the civilian authorities, including the emergency services, meaning that warning signs are not required on our vehicles.”