GENEVA (Reuters) - A European Union proposal to find spaces for 120,000 refugees will not work unless reception facilities are provided for tens of thousands at any time, the U.N. refugee agency said on Tuesday.

"A relocation program alone, at this stage in the crisis, will not be enough to stabilize the situation," UNHCR spokeswoman Melissa Fleming said.

UNHCR was no longer expecting a mandatory quota of relocation spaces allocated between EU countries, which it had hoped for, but urged EU leaders to back the 120,000 places as an emergency response, on top of 40,000 places for refugees who have arrived in Greece and Italy.

The new figure of 120,000 represents only 20 days of the daily average of 6,000 arrivals. Fleming said the UNHCR, which had called for an initial 200,000 places, expected the EU proposal would need to be expanded in the future.

Fleming said UNHCR chief Antonio Guterres had been on the phone to "every kind of leader who has some kind of role" including EU foreign ministers and prime ministers.

UNHCR has said a meeting of justice and home affairs ministers on Tuesday and a European Council meeting on Wednesday would be "crucially important" and possibly the EU's last chance to manage the crisis.

There had been a total of 477,906 "new sea arrivals" in Europe this year, Fleming said.

(Reporting by Tom Miles; Editing by Angus MacSwan)