Britain and other European Union countries must develop a common mass relocation plan for 200,000 refugees, the United Nations high commissioner for refugees has said.
Antonio Guterres said the EU is facing a defining moment and must "mobilise full force" towards a common approach to the migration crisis.
David Cameron is expected to come forward with new plans within the next few days which could see thousands more Syrian refugees come to the UK.
Speculation mounted that Britain could be on the brink of agreeing to take in a thousands vulnerable individuals from UN camps on the Syrian borders, rather than opening its doors to the desperate migrants who have poured into Europe across the Mediterranean.
But Mr Guterres said it is crucial that Europe develops a common approach to resettle refugees already within its borders and insisted "no country can refuse to do its part".
In a statement, Mr Guterres said: "Europe cannot go on responding to this crisis with a piecemeal or incremental approach.
"No country can do it alone, and no country can refuse to do its part. It is no surprise that, when a system is unbalanced and dysfunctional, everything gets blocked when the pressure mounts.
"This is a defining moment for the European Union, and it now has no other choice but to mobilise full force around this crisis.
"The only way to solve this problem is for the Union and all member states to implement a common strategy, based on responsibility, solidarity and trust."
The UN commissioner said the EU must develop urgent measures including emergency reception, assistance and registration capacity to stabilise the situation before agreeing a long term solution.
He said: "People who are found to have a valid protection claim in this initial screening must then benefit from a mass relocation programme, with the mandatory participation of all EU member states.
"A very preliminary estimate would indicate a potential need to increase relocation opportunities to as many as 200,000 places.
"This can only work if it goes hand-in-hand with adequate reception capacities, especially in Greece.
"Solidarity cannot be the responsibility of only a few EU member states."