Ukraine will not be able to take back significant territory from Russia without an increase in fighting power – including tanks and other heavy armour, Western officials have warned.
Officials said the two sides were currently too closely matched for the Ukrainians to be able to mount a successful offensive.
The chairman of the Commons Defence Committee Tobias Ellwood urged ministers to give the plan the green light, saying the West should be doing “far more” to support Ukraine.
President Volodymyr Zelensky has been pressing for months for Nato to provide his forces with US Abrams and German-built Leopard II battle tanks.
One Western official said that a reported Ukrainian call for 300 tanks was “not and unreasonable number” to create sufficient force “mass” for them to go on the offensive.
“One of the elements the Ukrainians lack in terms of capability are main battle tanks and armoured personnel carriers (APCs) in sufficient number to support offensive manoeuvre operations,” the official said.
“The Ukrainians won’t be unable to win back significant amounts of territory without changes to their force posture from last year. The force ratios between the Russians and the Ukrainians are too finely balanced.
“Something needs to break that deadlock, especially if they (the Ukrainians) are to win territory back and go on the offensive. Main battle tanks and APCs are part of that mix.”
Downing Street said no final decision had been taken on the supply of Challenger 2s, but confirmed that there had been discussion within the alliance on the issue of tanks.
“For several months, we’ve been working with partners around the provision of tanks and armoured vehicles, artillery and air defence,” the prime minister’s official spokesman said.
“The Prime Minister spoke to President Zelensky just last week about what can be most effective in helping Ukraine continue their progress.”
There have been concerns within Nato that supplying tanks could be regarded by the Russians as a further escalation of the conflict.
Poland, which operates Leopard IIs, has said that it would consider handing over some to the Ukrainians, but only in concert with other countries.
Mr Ellwood said he very much welcomed the fact that the UK was now looking at sending some of its heavy armour.
“This is our war, but we’ve left the Ukrainians to do the fighting,” he told BBC Radio 4’s World At One programme.
“It does show how far we’ve come in our willingness to look (Russian President Vladimir) Putin in the eye and not be spooked by his rhetoric, and we’re finally sending this much-needed serious hardware to Ukraine.
“Nato essentially has been benched. We should be doing far more to put this fire out and we’re not doing that.”
The latest discussion comes after the US last week promised the 50 Bradley tank-killing armoured vehicles in its biggest military assistance package to date for Ukraine.
Germany also announced it would supply around 40 Marder armoured personnel carriers and France promised wheeled AMX-10 RC tank destroyers.