Amnesty International has accused European leaders of "double speak" over a deal which will see Europe-bound migrants returned to Turkey.
The leading human rights charity said the deal failed to hide the EU's "dogged determination to turn its back on a global refugee crisis".
Under the plan, migrants arriving in Greece will be sent back to Turkey if their asylum claim is rejected.
In return, Turkey will receive aid and political concessions.
John Dalhuisen, Amnesty International's Director for Europe and Central Asia, said promises by the EU to respect international and European law "appear suspiciously like sugar-coating the cyanide pill that refugee protection in Europe has just been forced to swallow".
He added: "Guarantees to scrupulously respect international law are incompatible with the touted return to Turkey of all irregular migrants arriving on the Greek islands as of Sunday.
Scepticism hangs heavy in the air about a host of legal issues, and about whether the agreement can actually work in practice.
The idea at the heart of the deal - sending virtually all irregular migrants back to Turkey from the Greek islands - is the most controversial.
European leaders insist that everything will be in compliance with the law.
"It excludes any kind of collective expulsions," emphasised European Council President Donald Tusk.
The UN refugee agency (UNHCR) will take part in the scheme, but it is clearly uncomfortable with what has been agreed.
Turkey is "not a safe country for refugees and migrants", Mr Dalhuisen said, adding that any deal to return migrants based on claims it was would be "flawed, illegal and immoral".
It is hoped the plan, agreed at a summit in Brussels, will deter people from taking the often dangerous sea crossing from Turkey to Greece.
As part of the arrangement, EU countries will resettle Syrian migrants already living in Turkey. EU leaders have welcomed the agreement, but German Chancellor Angela Merkel warned of legal challenges to come.