Hundreds of workers at Britain’s biggest homelessness charity have started a three-day strike in a protest about working conditions and the treatment of vulnerable migrant rough sleepers whose details the charity shared with the Home Office.

Staff at St Mungo’s have gone on strike after a year-long dispute about the charity’s sickness policy and concern about the ratio of junior to more senior workers. Strikers are on picket lines in London, Bristol and Brighton. Union officials estimate that about 200 staff have joined the strike – the first at the charity since 2014.

Workers fear the staffing policy opens the door to a cheaper workforce on worse pay and terms and conditions. They claim that the working arrangements proposed by managers will undermine safety.

Staff have also expressed concern about the charity’s information sharing about migrant rough sleepers with the Home Office, a practice exposed by the Guardian.

Members of Unite rejected calls from the charity’s chief executive, Howard Sinclair, to suspend the strike because of coronavirus, instead saying they are following government guidance. They say that St Mungo’s chiefs rejected a compromise offer on staffing ratios.

Unite regional officer Tabusam Ahmed said: “St Mungo’s workers have tried their utmost to arrive at a reasonable settlement with their employer and have been rejected at every turn.

“For our members, the safety of their clients is their number one priority and they are taking strike action with very heavy hearts, but believe it is the only way to defend the future of St Mungo’s services.”

One anonymous St Mungo’s worker published a blog about why they were striking and described “a gutting feeling of disappointment, shame and guilt” when St Mungo’s admitted to sharing information with the Home Office about some migrant rough sleepers, after initially denying this. Last year the charity apologised for doing this.

St Mungo’s provides beds each night for 2,850 homeless people, in 120 hostels. Its annual turnover is £112m. It has accused Unite of opposing changes to how teams are staffed.

Sinclair said the charity had “done everything possible” to stop the strike and made compromises. He said the charity’s main focus was on organisation-wide planning around the coronavirus pandemic.

“We want Unite to call off their strike immediately,” he said. “With coronavirus, we know the focus of our staff, union or non-union members, will be even more on the welfare of our vulnerable clients.

“The right to life is more important than the right to strike, and organising pickets and rallies outside, while clients need support in services and when they are rough sleeping seems incredibly misjudged.”

Benjamin Morgan, who runs the EU homeless rights project at the Public Interest Law Centre, said: “This industrial action shows homelessness workers are making links between their own work conditions and the failure of managers to stand up for the rights of rough sleepers. St Mungo’s helped create a ‘hostile environment’ for homeless people by sharing personal data with the Home Office without consent.

“By doing so, they forfeited the trust of vulnerable migrants, making the work of outreach teams more difficult. Charities should never be sharing service users’ information for immigration enforcement purposes.”

A St Mungo’s spokesperson said on Monday: “St Mungo’s major challenge today is pandemic planning and all our resources should be focused on this. This is what our clients, staff and communities expect

“Unite officials have chosen to push ahead with their strike this week. We asked Unite to postpone their action, with no preconditions, which they have so far refused to do.

“This is really not the week to go out on strike and we ask Unite officials again to postpone during this unprecedented time. Our services have run as we expected today and we were able to provide staffing in line with our contingency plan. The safety and security of our clients remains our number one priority.”