France must act on the “dire” living conditions of refugees and migrants sleeping rough in Calais and stop systematically evicting people from tents in violation of the right to adequate housing, the United Nation’s housing envoy has said.

Leilani Farha, the UN’s special rapporteur for housing, highlighted the “harsh conditions” of an estimated 600-700 refugees and migrants homeless and sheltering in tents or makeshift camps on the northern French coast with “extremely limited access to emergency shelter”. Many have come to northern France because they hope to reach Britain.

Since the closure of a vast makeshift migrant camp in Calais in October 2016, hundreds of people are still sleeping rough along the coast in squalid conditions, many without proper access to sanitation.

Human rights groups in France say police in Calais repeatedly and forcibly evict people who are sleeping outside in wooded areas or by the sides of roads, often arriving in the early morning and confiscating their tents and sleeping bags, leaving them without any shelter.

Farha, who has analysed housing conditions across France, said: “The government of France must prohibit the repeated and systematic evictions of persons living in tents and informal settlements resulting in inhuman or degrading treatment.”

Leilani Farha, centre, visits homeless families in Paris. Farha said forced evictions by the French authorities ‘constitute cruel, inhuman or degrading treatment’. Photograph: Charles Platiau/Reuters

She told the Guardian: “People in Calais are being compelled to live in really dire circumstances, having already endured harrowing journeys there involving violence and extreme conditions … The evictions every 48 hours, which is what’s happening to those tent-dwellers, is unequivocally a violation of the right to adequate housing.”

She said since the spring of 2017 there has been a deliberate and systematic policy to evict people camping on privately owned grounds and in public places, including the sides of roads and under bridges. There were more than 200 evictions of tent encampments between January and March this year, according to local human rights organisations.

“These evictions usually take place in the morning with no advance notice provided to residents. Occasionally teargas has been employed against residents during the evictions,” Farha said. “Some witnesses I interviewed told me that they are being evicted every 48 hours. They also reported that they are not permitted to go back to their tents to retrieve them or to collect personal belongings. In fact, tents, sleeping bags and personal items are often destroyed or confiscated.”

In her preliminary findings Farha said the practices reported to her constituted a gross violation of right to adequate housing under international human rights law, and of the right to health, food, and physical integrity.

“The systematic and repeated nature of these forced evictions during winter suggest they also constitute cruel, inhuman or degrading treatment of one of the most vulnerable populations in France,” she said. “The systematic nature of repeated evictions by police forces is a source of extreme stress, anxiety and sleep deprivation – on a population that is already traumatised.”

When Calais’s notorious informal camp was destroyed by authorities in 2016, the French state imposed harsh conditions, and the UK put funding into security walls, in order to create what rights groups call an “inhospitable environment” aimed at putting off people from coming.

But this deterrence policy has left refugees and migrants sleeping rough in even more precarious and insanitary situations. Rights groups said although the numbers were lower, the living conditions worse, with only minimum access to water, food or toilets.

Farha said France, the sixth largest economy in the world, was a “beacon” in Europe, as the only country to have included the right to housing in domestic law. But it must now ensure that the right to housing was delivered, particularly in dealing with rising homelessness.

She also highlighted her “urgent concern” for people living in harsh conditions in squats and informal settlements across France, including refugees, migrants, Roma and Travellers. She had visited one empty office building in Toulouse inhabited by almost 300 people, including children, which was severely overcrowded, with insects everywhere and a sanitation system which was overwhelmed and flooding.

More than 16,000 people are estimated to live in 497 informal settlements in France, including shanty areas and makeshift groups of tents. One third are located in Greater Paris.