Illegal campers “put themselves and their families at great risk” by setting up camp near a former military firing range.

Traces of fire pits were found on the old range (pictured) and the illegal campers were seen in the river.

Following reports to Natural Resources Wales, vehicles were asked to move.

Guarding of the area will now be stepped up. All entry points will be closed, and extra barriers, fencing and signage erected.

Regular patrols will also be put in place to prevent any further illegal access, authorities said.

Dylan Williams, NRW operations manager in North Wales, said: “It is absolutely unimaginable to think that anyone would ignore the very clear warning signs that are all around Trawsfynydd Ranges to put themselves and their families at such great risk. What was witnessed here was absolutely shocking, and goes to show the extent of the issues that all parts of Wales are experiencing due to the significant increase in visitors following the easing of lockdown restrictions.

The incident came at the end of a weekend of increased presence by NRW officers at their North West Wales sites.

The patrols led to several vehicles in Coed y Brenin and Glasdir being removed, with warnings and advice issued to overnight visitors from as far afield as Bristol and London.

Mr Williams from NRW added: “We are grateful to the vast majority of visitors who leave no trace of their visit. Unfortunately some sites are seeing a significant increase in people who show little to no respect for the area.

“We’ve taken steps to increase patrols across our sites to tackle issues associated with overcrowding, fly parking, illegal camping and littering that have been seen across Wales over recent weeks. These measures have gone a long way in minimising such antisocial behaviour to keep our communities safe and protect the environment. We will continue to take further action where necessary.

“Ultimately, people wishing to visit our area must take personal responsibility to make sure they do so safely and with respect so that they don’t put themselves or others in danger.”

The issue of wild camping has been raised by MP Liz Saville Roberts. She said: “Most visitors are law-abiding and responsible and share the increasing frustration felt by local communities who have to deal with the mess left behind by an ignorant minority, unwilling to spend a few quid at some of our fantastic local campsites.

“I have heard several reports of human waste and litter left discarded and even fires illegally lit. All this has the potential to pollute our precious water resources, compromise public health as well as posing a big risk to wildlife and natural habitats.

“These groups should be reported to the authorities and swift action taken, not least as a robust deterrent to those of a similar mind-set. There is simply no excuse for it. I would appeal to those thinking of visiting the area to plan ahead, respect our communities and leave nothing but their footprints behind or risk inflicting lasting damage to an industry reliant on the forging of mutual respect between local communities and visitors.

The former military ranges southeast of Trawsfynydd were established in 1900, and became one of the country’s main ranges. They closed in the 1950s and the camp housed workers building the nuclear site.