An open air swimming lake in the eastern German city of Halle which has refused entry to bathers who don’t speak German has been told it must lift the ban or face possible legal action.

The Heidesee lake, a natural lake in a flooded former open-cast mine, recently introduced a check at the entrance to filter out visitors whose German was deemed not good enough to follow safety instructions.

Mathias Nobel, the lido’s manager, said he took the controversial step after a spate of cases in which visitors had ignored safety rules and lifeguards’ loudspeaker announcements.

“I’m responsible for the bathing here. If anything happened, everyone would point the finger at me. You can’t reverse death,” Nobel told local media.

The decision has led to anger and condemnation from critics who accused the venue of dressing up “a blanket entry barrier for entire population groups” as a safety precaution.

A spokesperson for the national anti-discrimination agency, which has been consulted on the row and could take legal action, said: “Imagine how much of a fuss there would be if German-speaking travellers in Mallorca had to prove their knowledge of Spanish or Catalan, or Arabic on the Red Sea, before they could go swimming?”

The authorities in Halle have demanded that Nobel drops the ban, saying it lacks proportionality. “The operator has to take into account the necessity of guaranteeing public access to the lido,” a city spokesperson said in a statement.

“The public character [of the lido] cannot be undermined by the implementation of house rules which amount to a blanket entry barrier for entire population groups.”

The authorities added: “Any action that might be perceived as xenophobic could damage the city’s reputation.”

Germany’s life saving association, the DLRG, said in a statement it firmly distanced itself from the Heidesee ban.

In Germany, as in the US and other western countries, swimming pools have become unlikely focal points for racial tensions and rows about immigration, stirred up by the far right.

Halle is in the state of Saxony-Anhalt, which has an election in September. The far-right anti-immigrant Alternative für Deutschland (AfD), which is leading the polls with about 42%, has seized on the row.

“Our public swimming pools, once safe havens of recreation, are increasingly becoming genuine danger zones under the misguided policies of the established parties,” the party wrote in a Facebook post. “When private operators are forced to implement their own language controls to ensure the safety of swimmers, the state’s loss of control has definitively reached the heart of our society.”

It uploaded a poster to social media with the slogan: “Those who don’t understand German, stay out.”

The party has often used images of swimming pools in its posters and campaign literature, most famously in a colouring book for children that contained racist and xenophobic stereotypes. It included one image of women in full-body veils swimming in a pool while men with knives and pistols linger in the background.

Nobel, a trained lifeguard, denied the measure was racist or xenophobic, saying it was particularly important that swimmers understood “the German bathing rules” at his pool because the lake is deeper than a conventional swimming pool and has a steeply sloping shoreline.

The city authorities have called on him to find “milder ways” to deal with the communication issues, such as using pictograms that are universally understood or displaying safety messages in other languages.