European leaders have been warned two emergency meetings could be the "last opportunity" to thrash out an effective response to the migrant crisis.
Home Secretary Theresa May is attending talks with her counterparts from other EU states in Brussels today, while David Cameron will meet fellow leaders tomorrow.
The most crucial issue at stake will be whether politicians can resolve disagreements over a plan to relocate 120,000 asylum seekers from Italy, Greece and Hungary.
Britain has vowed not to take part in any resettlement scheme, and the Prime Minister is expected to use tomorrow's summit to urge EU leaders to do more in the region. The UK has agreed to take 20,000 Syrian refugees from camps in neighbouring countries.
Ahead of the meetings, the United Nations refugee agency said the situation is becoming "increasingly chaotic and unpredictable" and warned that a relocation scheme alone "at this stage in the crisis, will not be enough to stabilise the situation".
UNHCR spokeswoman Melissa Fleming said: "This may be the last opportunity for a coherent European response to manage a crisis that is increasing suffering and exploitation of refugees and migrants and tension between countries."
The agency's data shows that a total of 477,906 people have arrived in Europe by sea this year.
Ms Fleming said it is "essential" that the 120,000 additional places are approved this week for any relocation programme to be considered "credible" but claimed that any scheme cannot be effectively implemented without the creation of "adequate reception facilities" in countries where refugees and migrants enter Europe.
"With an average of 6,000 persons arriving every day on European shores, this requires a massive investment," she said. "Many tens of thousands of people are likely to require shelter and assistance at reception areas at any given time.
"The emergency can only be addressed through a holistic and comprehensive approach, with all EU Member States working together in a constructive manner."
The UNHCR called for states to take four steps to ease the crisis:
:: Strong support for the immediate creation of reception facilities in Greece and the expansion of existing centres in Italy.
:: Processes to be started immediately to relocate 40,000 people from Greece and Italy.
:: Strengthened mechanisms for the humane return of those not granted international protection.
:: Urgent measures to stabilise the situation including additional humanitarian funding for countries hosting large refugee populations.
Labour MP Keith Vaz, chairman of the home affairs committee, called for the EU to end "the internal squabbling and bickering which only serves to aggravate the already desperate refugee situation".
He added: "This has become a stain on the history of the continent, and a betrayal of the values of the EU which are based on decency and humanity. European governments need to act now to stop this senseless herding of vulnerable people and passing them from country to country.
"There have been successive failures of European governments to bring order to chaos.
"A summit is long overdue and leaders must finally produce coordinated and concrete measures before greater cracks start to emerge in EU home affairs policy on this critical issue."