The British Army will become temporarily weaker and less able to combat Russia after giving away tanks and artillery to Ukraine, its top general has said in an unusually blunt admission.
General Sir Patrick Sanders told his troops that the decision to help the Ukrainian military defeat Vladimir Putin's invasion would make the UK safer.
But he also stressed the "vital" need to restore his army's warfighting capability.
The comments were made in an internal video message to the Army that appeared designed to put pressure on the Treasury to commit more funding to defence. Sky News has seen a copy of the transcript.
"Wars are won and lost on land," the chief of the general staff said in his statement, which was issued after Ben Wallace, the defence secretary, confirmed to parliament on Monday that the UK would be sending 14 Challenger 2 tanks as well as artillery guns to Ukraine as part of a significant new package of military support.
"Ukraine needs our tanks and guns now. I know they will put them to good use. And there can be no better cause," General Sanders said.
He said the UK pledge would encourage other allies to follow suit with more "battle-winning" weapons in greater numbers.
However, the well-regarded officer admitted: "Giving away these capabilities will leave us temporarily weaker as an army, there is no denying it.
The top general continued: "There is no doubt that our choice will impact on our ability to mobilise the army against the acute and enduring threat Russia presents and meet our NATO obligations.
"Our tank crews and gunners will feel the impact the most, but the decision also brings the opportunity to accelerate the modernisation and transformation of the army ahead of Russia."
Rishi Sunak's government is refreshing a sweeping review of UK defence and security with the army in need of billions of pounds more in funding to fulfil plans to transform with upgraded tanks and artillery as well invest in vital weapons such as long-range missiles and air defence systems.
But the prime minister has yet to commit even to maintaining defence spending flat in real terms - when the impact of inflation is taken into account.
A failure to inject significant new money into the defence budget would result in real-term cuts.
The results of the refresh are set to be published in March around the time of the Budget.
"It is vital that we restore and enhance the army's warfighting capability at pace to reinforce our combat credibility and retain our position as the leading European ally in NATO," General Sanders said.
"To that end, I am also determined that we do our utmost to maintain the currency and competency of those affected by our decision to gift these platforms.
"The government is committed to a modernised army that has learnt the lessons of the war in Ukraine and emerges from a period of accelerated investment more lethal, more survivable and able to fight more effectively as part of a joint force.
"This is the army our nation needs; this is the army you deserve."