The UK is considering sending tanks to Poland as part of a move that would then see Poland supply Ukraine with Soviet-era tanks.
Boris Johnson said 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 2 battle tanks - the British Army's main battle tank - to Poland was being "looked at".
The Prime Minister also said Western allies are preparing to offer Ukraine a series of "security guarantees" which should make the country "impregnable" to a future Russian invasion.
Speaking in the Indian capital, New Delhi, 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.
While he said it will not be the same as the NATO Article 5 guarantee – in which an attack on one member state is considered an attack on all – he hoped it would offer "deterrence by denial".
"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.
In its latest intelligence update, the UK Ministry of Defence (MOD) said Mr Putin's decision to blockade Azovstal steel works in the besieged city of Mariupol was intended to release troops for the Russian offensive in the Donbas region.
An estimated 2,000 Ukrainian troops remain holed up in the vast plant, where they have been holding out against numerically-superior Russia for weeks.
Mr Johnson said the situation in the region remains "unpredictable", underlying the need to maintain support for Ukraine.
"We have got to look at what more we can do militarily. We have got to keep intensifying economic sanctions. We want to make sure there is wave upon wave of intensifying pressure on Putin," he said.