THERESA May is being urged the scrap a policy which allows asylum seekers to be locked up without limit of time.

The call is being made by the SNP after two of its MPs – Stuart McDonald and Gavin Newlands – last week visited Scotland’s only detention centre at Dungavel in South Lanarkshire.

It comes weeks after submissions to a review on the welfare in detention of vulnerable people reported not knowing how long they were being held exacerbated any mental health problems they had.

The Shaw report, published last month, found many people seeking refuge in the UK were traumatised and vulnerable, while the UK is one of the few EU countries where people can be detained indefinitely.

McDonald called for the Prime Minister to end the “without limit of time” provision as part of wider reforms to make the UK’s immigration policy more humane.

“Many people would be astonished to know that each year around 30,000 people are detained in what are essentially private prisons, simply for the administrative convenience of the Home Office and without limit of time,” said McDonald, the SNP’s immigration spokesman.

“As the recent independent review by Stephen Shaw has pointed out, far too many vulnerable people are still being detained.”

He added: “We need much stronger independent oversight of decisions to detain people, better screening to ensure vulnerable people are not detained and a time limit on how long people are detained – not knowing is terrible for people’s mental health.”

McDonald continued: “It is time to radically reform detention policy – because locking up vulnerable people for little apparent purpose is an affront to the rule of law.”

The Shaw review found more than 200 detainees were on watch for self harm last year at the Dungavel detention centre which holds asylum seekers while the Home Office decides whether they can remain in the UK.

It also reported that a secure unit at the centre was “wholly unsuitable” for people with mental health conditions and that health facilities were “not clean”.

The report by Stephen Shaw, a former prison ombudsman for England and Wales, also raised concerns over the time staff took to section people with serious mental health issues.

Dungavel House is run by the GEO Group UK Ltd, a subsidiary of US company GEO Group, which runs immigration detention centres in America. Dungavel can hold a maximum of 249 people.

Shaw’s report — a follow up to his review in January 2016 on the UK’s immigration removal centres also revealed that 206 detainees were at risk of self harm last year and that 10 people were returned to the detention centre after being sectioned. There were 42 complaints received at Dungavel in 2017. Shaw said the centre’s secure unit was “bleak and excessively bare”.

The UK Government has been at the centre of criticism over its hostile environment policy on immigration to the country.

In recent months people targeted by immigration services include 10-year-old orphan Giorgi Kakava, who was threatened with deportation from Glasgow to Georgia, and Solomon Getenet Yitbarek. Yitbarek, 27, had already been deported to Ethiopia, where his father and brother were tortured and later died, when a Scottish judge ruled the Home Office had acted unlawfully and that he should be brought back to the UK.

A Home Office spokesman could not be contacted yesterday for a comment.