Hundreds of asylum seekers in Glasgow are facing fresh threats of eviction after the private housing provider Serco announced it was restarting its controversial lock-change policy.

Serco first announced plans last July to evict 300 asylum seekers who have been told they cannot stay in the UK, but put the plans on hold following cross-party and cross-sector condemnation.

Govan Law Centre lodged a legal challenge to the lock-change procedures, which the company describes as its “Move On Protocol”, but lost when Scotland’s highest court, the court of session, ruled in April the protocol was not illegal.

After Serco’s announcement on Wednesday, Glasgow city council immediately warned of an “imminent homelessness crisis”, while refugee charities expressed outrage they had not been given advance warning of the plan and accused Serco of spreading “fear and anxiety”.

Both the leader of Glasgow city council, Susan Aitken, and the Scottish government’s communities secretary, Aileen Campbell, have written to the UK immigration minister, Caroline Nokes, demanding she take urgent action.

Graham O’Neill, policy manager at the Scottish Refugee Council, said: “We are outraged at Serco’s plan to initiate lock-change evictions in the coming weeks and months. For hundreds of men and women in Glasgow this will mean forced eviction from their only source of accommodation and safety in Scotland.”

Warning the plans would result in “extreme human suffering on a mass scale with all the immediate adverse health and exploitation risks that ensue”, O’Neill added: “Serco and the Home Office are aware of several ongoing legal challenges against the lock-change policy. Scots legal process has not been exhausted and we don’t believe the law in this regard has been definitively clarified.”

He said agencies working with people refused refugee protection knew nothing about the plan, adding: “Once again [Serco’s] public announcement has spread fear and anxiety among refugee communities.”

Serco, which lost the Home Office contract in Scotland earlier this year, said it was “not a step we have taken lightly”. It guaranteed that no more than 30 people would be issued with lock-changing notices in any one week, that tenants would be given at least 21 days’ notice to make alternative arrangements and that no children would be left homeless.

Serco also promised to donate up to £150,000 to charities supporting homeless people in Glasgow. This is the equivalent of the Scottish Refugee Council’s destitution grant of £70 a fortnight, which does not include accommodation costs, for the 300 individuals affected for just over three months.

Julia Rogers, Serco’s managing director for immigration, said: “We very much regret the distress this will cause, but hope that it will be understood that we cannot be expected to provide free housing indefinitely to hundreds of people who have been unsuccessful in their asylum claims and most of whom have no legal right to remain in the UK.”

In her letter to Nokes, Aitken points out that the “no recourse to public funds” policy “renders local authorities powerless to respond and unable to provide the necessary support for many of those who will be affected”.

“It is a sorry and utterly unacceptable state of affairs when a UK government contract legally obliges its contractor to force people from their homes and leave public servants to choose between either breaking the law or allowing mass destitution on the streets of our city.”

Aitken continued: “When we spoke about this last year, I asked you to give me an undertaking that future lock-changes would not take place, and warned that if that did not happen we would simply repeat the cycle of me having to protest to you about an imminent homelessness crisis in my city. I deeply regret that this has come to pass.”

Campbell called on the Home Office to “live up to its responsibilities” in her own letter to Nokes.

She wrote: “It is not acceptable to leave the asylum accommodation provider to deal with the inevitable results of a flawed system, and to wash your hands of the consequences.”

A Home Office spokesperson said: “The Home Office takes the wellbeing of asylum seekers and the local communities in which they live extremely seriously.”