Plans to take failed asylum seekers' children into government care if they refuse to leave the country may breach the Human Rights Act, campaigners say.
An asylum amnesty for families was announced last month
Under the proposal, due to be announced on Wednesday, parents will be told to take paid-for "voluntary" flights home or lose their benefits and their children.
A Home Office spokesman said the move would protect children from action taken against their parents.
But campaigners say it could lead to costly court cases if challenged.
It is thought up to 2,000 children could be affected by the plan, to be included in the Queen's Speech as part of the controversial Asylum Bill.
The Home Office denied the move was draconian, insisting it was the "logical outcome of a managed migration policy".
But pressure group Refugee Action called the plan "inhumane and shameful in the way it targets children as a way of coercing parents to return".
Spokesman Stephen Rylance said it could breach family rights under the Human Rights Act, possibly sparking expensive legal action.
The policy meant that children who had already suffered the trauma of fleeing their homeland would then be put through the trauma of being separated from their parents, he said.
However, he told BBC News Online that parents could be "absolutely terrified" of returning to their own country and be forced into "desperate measures".
"Parents may believe that it is better for their children to be looked after by the state, and if they are genuinely afraid of returning then they may disappear altogether and have no contact with authorities."
Human rights group Liberty and the Refugee Council joined Refugee Action in attacking the plan.
The Conservatives also criticised it, with shadow home secretary David Davis saying it was an "odd thing" for the government to do.
But the Home Office issued a statement on Sunday defending its proposal.
It was "the only logical way of dealing with people who have no right to be in the country and therefore no right to public funding or accommodation,
but who are simply refusing the organised offer of a paid return home", a spokesman said.
"The policy is not designed to make families destitute and we do not believe many, if any, people would put their children in this position.
"In rare cases where it is necessary to end support we would not want children to be made destitute as a result of the actions of their parents so provision would be made to take them into care."
But Liberty spokesman Barry Hugill said: "What this is all about is the government looking for sound bites to appear as if it's tough on a perceived problem.
"I think it is all rather distasteful to start concentrating on children in the hope that you will get good headlines saying how tough you are. It's rather unpleasant."
The Refugee Council said that "breaking up families harms children and should be done only when there is absolutely no alternative".
"The government should abandon this plan and work instead to improve the efficiency and effectiveness of the asylum system," a spokesman said.
He told BBC News Online the plan would be the fifth change to asylum legislation in the past decade, and called on the government to put in place "foundations that can endure".
Shadow home secretary David Davis said the Conservatives supported the idea that benefits should be stopped with the failure on a would-be asylum seeker's appeal.
But in an interview to for BBC Radio 4's Westminster Hour he added: "I would have thought the next step frankly would be to send them [failed asylum seekers] home rather than saying 'we will take your children off you if you don't go'."
News of the plan comes after Home Secretary David Blunkett announced in October that up to 15,000 refugee families would be allowed to live and work in the UK in the biggest-ever asylum amnesty.
Other reform measures include the prosecution of asylum seekers arriving in the UK without travel documents, restricting the number of appeals to one, and a crackdown on unqualified legal advisers who "abuse" the system.