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Defence Secretary Ben Wallace has said people without military training should not travel to Ukraine to fight the Russian invasion.
On Sunday, the Foreign Secretary Liz Truss said she supported those who wanted to join an international force.
Ukraine's president has said his country is setting up a volunteers legion for foreigners wishing to fight.
Mr Wallace warned the situation was "dangerous" and that there were other ways to help the country.
Ukraine President Volodmyr Zelensky made the call for volunteers on Sunday saying: "Everyone who wants to join the defence of security in Europe and the world may come and stand shoulder to shoulder with Ukrainians against the invaders of the 21 Century."
Asked on BBC One's Sunday Morning programme if she would support individuals from Britain going over to Ukraine to help in the fight, Ms Truss said: "I do support that, and of course that is something that people can make their own decisions about.
"The people of Ukraine are fighting for freedom and democracy not just for Ukraine, but for whole of Europe because that's what President Putin is challenging."
Her comments were criticised by Conservative MP and chair of the defence committee Tobias Ellwood, who tweeted: "We say no to tasking the Nato alliance - but now endorse UK civilians, with no military training, to go and fight in Ukraine!"
"Please do not go if you have no combat experience. You may get yourself and others who have to look after you, killed."
Russia attacks Ukraine: More coverage
Labour's Clive Lewis said "untrained people going to a country where they can't even read a road sign" would be "a liability" adding: "They'll get themselves killed, they'll get other people killed."
Defending the foreign secretary, Mr Wallace said she had not been suggesting untrained people should travel to Ukraine.
He said people needed to be trained and to have experience, but added that those currently serving in the British Army should not go to Ukraine.
For those who want to help, he advised donating through the Ukrainian embassy website.
The UK government has stressed that British troops will not be sent to fight on the ground.