WARSAW/LONDON, March 1 (Reuters) - Prime Minister Boris Johnson warned on Tuesday about the growing humanitarian crisis caused by Russia's invasion of Ukraine, saying refugee numbers could reach millions, with possibly more than 200,000 coming to Britain to join family members.

Less than a week after Moscow launched a full-scale invasion of Ukraine, western leaders are looking at ways to help the hundreds of thousands of Ukrainians who have left their homeland.

Poland has estimated that about 350,000 people have crossed its border from Ukraine since last Thursday, while the European Union has emphasised the need to prepare for millions of refugees entering the bloc.