More than 4 million refugees have now fled Ukraine since Russia launched its invasion, according to the UN refugee agency (UNHCR).
With a population of about 44 million, this means that nearly a tenth of the Ukrainian population has left the country in five weeks.
Some 2.3 million have sought sanctuary in neighbouring Poland, and more than 600,000 are in Romania, data provided by UNHCR showed.
Moldova and Hungary have accepted 387,000 and 364,000 people respectively, while Slovakia has welcomed 281,000.
Around 350,000 people have also crossed into Russia, although authorities there have been accused of forcibly relocating thousands of civilians from Mariupol over the border.
The movement of people out of Ukraine since Vladimir Putin’s invasion began on 24 February has triggered the largest refugee crisis in Europe since the Second World War, according to the UN.
Filippo Grandi, the UN High Commissioner for Refugees, tweeted confirmation of the bleak milestone and said he would be in the western Ukrainian city of Lviv to discuss ways to help those "displaced by this senseless war".
In a note accompanying the data, UNHCR said: “The escalation of conflict in Ukraine has caused destruction of civilian infrastructure and civilian casualties and has forced people to flee their homes seeking safety, protection and assistance.
“In the first week, more than a million refugees from Ukraine crossed borders into neighbouring countries, and many more are on the move both inside and outside the country.
“In light of the emergency and paramount humanitarian needs of refugees from Ukraine, an inter-agency regional refugee response is being carried out, in support of refugee-hosting countries’ efforts.”
With millions of uprooted Ukrainians now spread across Europe, the UK is under mounting pressure to play its part in the humanitarian effort.
However, there have been just 2,700 visas granted to people wanting to come to the UK under the Homes for Ukraine scheme, latest figures show.
The scheme opened on 14 March, with the aim of allowing individuals, charities, community groups and businesses to bring Ukrainians - including those with no family ties to the UK - to safety.
Britain retains a visa requirement on security grounds while other European countries have waived checks in response to the humanitarian crisis created by Russia’s war.
The total number of applications from Ukrainians wanting to come to the UK was 59,500 as of 29 March, the Home Office said.
This includes 28,300 under the Homes for Ukraine Scheme and 31,200 under the separate Ukraine Family Scheme, which allows Ukrainians to join relatives living in the UK.
There have been 25,500 visas issued altogether, including 2,700 under the sponsorship scheme and 22,800 under the family scheme.
On Tuesday, UNHCR warned that thousands of Ukrainians who were already in the UK at the time of the Russian invasion risk being left in a “precarious state” unless the government moves to quickly grant them leave to remain.
Workers on short-term work visas are among those affected, the agency said, in particular agricultural workers who are no longer needed by their employers or who cannot continue in that line of work.
“While it’s encouraging to see the UK take steps to welcome thousands of Ukrainian refugees to the UK, we mustn’t forget the thousands that were already here when the war started in February,” said Larry Bottinick, Acting UNHCR Representative to the UK.
“They too can’t now return and need legal security in order to become self-sufficient.”
UNHCR is also calling on the UK to extend family reunion rights to all Ukrainians in the country. The agency says not all Ukrainians currently in the UK, for example those on short term visas, are able to bring their family out through this scheme.
