Speaking in the Indian capital, New Delhi, Prime Ministry of UK Boris Johnson said, that tanks would be sent to "backfill" in Poland so Warsaw can release T-72 tanks - with which Ukrainian troops are familiar - to Kyiv.
A defence source said sending Challenger II battle tanks - the British Army's main battle tank - to Poland was being "looked at".
Read more: Slovenia to Hand Over M-84 Tanks to Ukraine, Get German Leopard and Marder in Return
Mr Johnson said it is essential to step up immediate military support to Kyiv, warning there is a "realistic possibility" the conflict could drag on for a "long period".
He also said a long-term vision for Ukraine's place in the future "security architecture" of Europe needs to be developed.
"What the Ukrainians want – and I think are now going to get – is a collection of guarantees from like-minded countries about what we can do to back them up with weaponry, with training, and with intelligence-sharing," he said.
"It will, I hope, enable the Ukrainians to offer deterrence by denial and make sure their territory is so fortified as to be impregnable to further attack from Russia. That is what we need to do."
The Prime Minister said the improving security situation around Kyiv means that Britain will be able to reopen its embassy there next week.
However, he accepted a Western intelligence assessment that the conflict could continue for most of the rest of the year, potentially ending with a victory for Russian President Vladimir Putin.
Read more: Slovenia, Spain and Denmark to Send Additional Military Assistance to Ukraine
