Keir Starmer has said Brits are offering "all sorts of support" to Ukrainians on the ground after making a surprise trip to Kyiv.
The Government advises not going to Ukraine, saying such activities could amount to "offences" under UK law.
When asked on BBC Radio 2's Jeremy Vine programme if it was "good that British people are going out to fight", Mr Starmer said: "I think a lot of British people are going out to offer all sorts of support.