Western countries say they fear Russia is preparing a pretext for a new assault on Ukraine, which it invaded in 2014.
Moscow denies any plans for an attack, but has said it could take unspecified military action unless the West agrees to a list of demands, including banning Ukraine from ever joining NATO. Talks last week ended with no breakthrough. Kyiv has asked Western countries for arms to help it protect itself.
"We have taken the decision to supply Ukraine with light anti-armour defensive weapon systems," British Defence Secretary Ben Wallace told parliament, saying the first systems were already delivered on Monday and a small number of British personnel would provide training for a short period of time.
He did not specify the number or type of weapons that were being sent, but said: "They are not strategic weapons and pose no threat to Russia. They are to use in self-defence."
"These are short-range .... but nevertheless it would make people pause and think what they were doing and if tanks were to roll into Ukraine, invade it, then they would be part of the defence mechanism."
Ukraine's defence minister welcomed Wallace's announcement.
"Ukraine highly appreciates Britain's decision to provide a new security package with light, anti-armour, defensive weapon systems!" Oleksii Reznikov said in a tweet.
Britain has previously warned Russia of severe consequences if it launched a new military assault on Ukraine, while offering financing to enhance Ukraine's naval capabilities.
Wallace said he had invited Russian Defence Minister Sergei Shoigu to visit London in the next few weeks to discuss the crisis, though he did not know whether the Russians would accept.
"The current gap is wide but not unbridgeable," Wallace said, voicing the hope that diplomacy would prevail and adding, "It is President (Vladimir) Putin's choice."
(Reporting by Kylie MacLellan and Alistair Smout; additional reporting by Natalia Zinets and Matthias Williams; Editing by Kate Holton, Peter Graff and Howard Goller)