The European Union could be forced to bring back border controls because of the migrant crisis, says German Chancellor Angela Merkel.
She said the Schengen zone, which allows passport-free travel across mainland Europe, could not continue in its present form unless other EU states took their share of migrants.
Merkel said she did not want freedom of movement rules to be tightened, but the issue would arise if responsibility for accepting migrants was not shared.
Her comments indicate that European leaders are beginning to question whether the EU can continue to exist with open borders as it struggles to cope with hundreds of thousands of migrants from Africa and the Middle East.
Germany is set to receive 800,000 asylum seekers this year, four times the total for 2014 and more than any other EU country. Merkel said: "Europe as a whole needs to move. Member states must share responsibility for asylum-seeking refugees. If we don't arrive at a fair distribution then the issue of Schengen will arise - we do not want that."
A total of 3650 migrants reached Vienna by train in one day, this year's biggest daily number, after Hungarian authorities allowed people stuck in refugee camps to leave Budapest despite many not having EU visas. "We are still in the process of verifying how many of them are actually asylum-seekers," Austrian police spokesman Patrick Maierhofer said.
Many of the migrants slept at Vienna's Westbahnhof station, hoping to continue their journey on to Germany. Hungarian authorities were last night stopping all trains from leaving Budapest's main terminal and clearing the station as hundreds of migrants tried to board trains to Austria and Germany. An announcement over loudspeakers said the measure would be in effect for an undetermined length of time. Scuffles broke out earlier among some of the migrants as they pushed towards a train scheduled to leave for Vienna and Munich, and were blocked by police.
Yesterday Austria started to inspect cars and trucks travelling from Hungary towards Vienna. Queues up to 48km long grew along the Austro-Hungarian border, thanks to the spot checks. "The main aim is to target smuggling gangs," said a police spokesman. Austrian officials detained 200 migrants and five people smugglers. So far this year, the Hungarian Government has reported more than 140,000 migrants entering from Serbia.
The International Crisis Group said the Greek island of Lesbos was being pushed beyond breaking point by a record number of arrivals in the past few days, with 13,000 refugees now crammed on to the tiny island.
On Friday the leaders of Hungary, Poland, the Czech Republic and Slovakia will meet in Prague and are expected to reject quotas - knowing most migrants aim for the wealthier northern countries.
• The EU is to grant France an extra €5 million to turn the tent city in Calais known as "The Jungle" into a "refugee camp" for migrants seeking to reach Britain illegally. The camp would house 1500 migrants. About 4000 live in squalid conditions at the site.
• Ten thousand Icelanders have offered to welcome Syrian refugees into their homes, as part of a Facebook campaign launched by author Bryndis Bjorgvinsdottir after the Government said it would take in 50. The Government is looking at increasing its quota.
• About 20,000 people have marched in Vienna against ill-treatment of migrants, holding up banners reading "Refugees welcome" and "I don't want Europe to be a mass grave".
• Video footage has emerged of the Spanish coastguard pursuing men on jetskis who smuggle migrants to Europe. Officers with go-pros on their heads use speedboats to hunt down smugglers. Six jetskis were stopped last week near Tarifa. The traffickers can pack three immigrants on to the backs of each jetski as they cross the Straits of Gibraltar. Paramilitary police rescued two migrants and a pilot after their jetski overturned.
- Telegraph Group Ltd, AFP, AP