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.
"Some of them we saw yesterday and this morning.
"Obviously, if people are going out to fight, it has got to be very carefully done because you've got the Ukrainian forces there.
"They are disciplined, they are professional, they are working to a strategic plan and nothing should be done that gets in the way of that.
"So, Ukraine wants all the support it can get but nothing that undermines their strategy, their tactics."
Mr Starmer emphasised the strength of the Ukrainian army and emphasised their requests for weapons, training - and ultimately justice in the face of Russia's horrific war crimes.
The Labour leader said: "As we come up to the one year anniversary, I remember that back then it was anticipated that the Russians would take Kyiv within a week.
"And that has obviously, the position has been completely changed, through the strategy and tactics of Ukrainian forces and I don't think anything should cut across that.
"What they want is reinforcement, better training of their own people and the weaponry to go with it.
"If I took anything away from those awful bits of evidence of atrocities in the outskirts of Kyiv, it was this sense of justice that somebody has to stand trial for the atrocities, the cowardly war crime killing of their families, their communities.
“As we look to reparations as well as war crimes, Russia has to pay. There's a lot of money tied up in assets that have been frozen and what the Ukrainians want is us to use that money as compensation so they can rebuild.
“I looked at the residential blocks that have been bombed, these are family blocks, they're trying to rebuild as the conflict goes on. They desperately need that help now."
Mr Starmer yesterday met with President Volodymyr Zelensky in Kyiv to show his “unwavering support” for Ukraine against Russian aggression.
It comes following the historic visit of Ukraine's President Volodymyr Zelensky to the UK last week.
A one-minute silence will be held on February 24 across the UK to mark a year since Russia ’s barbaric invasion of Ukraine.
The Foreign Office's official guidance on the war in Ukraine says: "If you travel to Ukraine to fight, or to assist others engaged in the war, your activities may amount to offences under UK legislation and you could be prosecuted on your return to the UK.
"British nationals fighting in Ukraine have been killed or captured. Additionally, British nationals undertaking humanitarian work have been detained. The risk to life, or of mistreatment, is high. Consular support in these circumstances will be very limited."
Liz Truss sparked confusion as the conflict broke out last year when she remarked on live TV that she'd support people who decide to flock to the eastern European state to help Ukrainians fight, despite guidance from her own Government advising them not to travel there.
