By Natalie Bloomer and Samir Jeraj
Members of parliament have been using a Home Office hotline and webpage to report people for immigration enforcement, Politics.co.uk can reveal.
In response to a Freedom of Information request, the Home Office confirmed that it received 482 tip-offs from MPs between 2014 and 2016.
The news raises the possibility that MPs are reporting their own constituents to the authorities when they have come to them for help.
The Green party today hit out at the practice calling it a "fundamental betrayal of trust" and called for the MPs involved to come clean about how they came about the information they reported.
"This appears to be a fundamental betrayal of trust and duty of care if elected representatives have been tipping off the Home Office about people they suspect to be undocumented migrants," party co-leader Jonathan Bartley said.
"Members of parliament should support, defend and advocate for all those living in their local communities, not have them dragged away to indefinite detention and potential deportation.
"Serious questions must now be asked about how the Home Office's 'Big Brother' system is being used by MPs.
"Those involved must come clean about how they came by the information upon which they have acted. Has personal information been abused? Have they acted upon information given in good faith by alleged undocumented migrants themselves? Have they abused the privilege of their office?"
The news comes amid growing concerns about the impact of Theresa May's "hostile environment" for undocumented migrants, which has led to some schools and hospitals sharing information with the Home Office.
Last week, Politics.co.uk reported that London mayor Sadiq Khan felt police officers were "duty bound" to report both victims and witnesses of crime to the Home Office for immigration enforcement.
The Joint Council for the Welfare of Immigrants called today's news "extremely concerning".
“All constituents have a right to petition their MPs and should have confidence that they will not betray their interests," the legal and policy director Chai Patel said.
"It is extremely concerning that members of parliament appear to be sharing information with the Home Office in these circumstances. With massive cuts to legal aid many migrants have nowhere else to turn to find someone who can advise them and represent their interests.
"We need to get to the bottom of what information MPs are giving the Home Office and in what context.”
Fizza Qureshi, director of the Migrants' Rights Network, said:
"If parliamentarians are reporting their constituents, it demonstrates how deep the 'hostile environment' is seeping into our society.
"You would hope that MPs would advocate on behalf of their constituents rather than report them to the Home Office."
A Home Office spokesperson said:
"Like any member of the public, MPs can use the form on GOV.UK to report an immigration crime. They would only be identified as an MP if they declared themselves on the form, which allows someone remain anonymous."