Protesters stood outside the 'naked swim' event at Blackpool's Sandcastle water park

The private event, held by British Naturism, saw registered families invited to swim in the nude on Saturday.

However, the group Templar Knights, which states on its Facebook page that its members are “guardians of children”, protested outside the water park, confronting some of those who took part.

Another group, which uses the Facebook page Fleetwood Enforcers UK, said its members had also attended to protest the "disgusting" event. Police were present but made no arrests.

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Protesters stood outside the 'naked swim' event at Blackpool's Sandcastle water park

Videos from the protest have been viewed more than 200,000 times.

The Templar Knights group, which is based in South Yorkshire according to its Facebook page, broadcast a live video on the social media site in which a small group of protesters took exception to the number of single men entering without children.

But today British Naturism said it had a “robust safeguarding policy” and the private event was open only to pre-registered members.

In a statement, British Naturism said: “British Naturism is an organisation that has been in existence for over 50 years with many of our clubs being even older. We have a membership of 9,000 over 18s.

“We have a robust child and vulnerable adults safeguarding policy. It was developed in conjunction with the NSPCC and is reviewed annually. We take our safeguarding and health and safety responsibility seriously and review our practices in advance of every event we run.

“Children at our events are always supervised by their parents and not allowed to attend unaccompanied – unlike schools, churches, scouts, play areas and other venues where parents opt to leave their children in the care of others.

“Naturist swims, most of which are in public venues hired for the purpose, have been run for many decades and new venues are being added all the time. The Sandcastle session is not an isolated event – naturist swims are being successfully – and safely – run all around the country throughout the year.”

But Emma Bennett, who filmed the broadcast for Templar Knights, expressed concerns at this claim. She said: “A naked family swim cannot be properly safeguarded.

"How can the police and NSPCC guarantee that nothing is going to happen underwater?"

A Sandcastle spokesman said: “At Sandcastle Waterpark we take safeguarding extremely seriously, as such all activities within our Waterpark are considered from a safeguarding perspective. The naturist event (including family naturists) was a private event and no one can walk in off the street without pre-registration.

“Any children that might attend on the night are with their parents or guardians, which is part of the safeguarding policies of The British Naturist Society (their safeguarding policy is available on their website). As with all activities at Sandcastle Waterpark we ensure that there is lifeguard supervision throughout the waterpark and all areas - except toilets and male and female changing rooms - have comprehensive CCTV coverage, at all times.”

A Lancashire Police spokesman said: "A small number of people attended the Sandcastle at Blackpool on Saturday evening to protest at a nudist swimming event.

"Police attended to ensure that there was no breach of the peace. No arrests were made."

British Naturism has existed since 1964 and has around 13,000 members.