On Thursday night Sir Keir told the Nato summit that he had “reaffirmed our unshakeable support for Ukraine’s ultimate victory.”

He said the alternative was “unthinkable” as he described Russia using “some of the deadliest weapons in its arsenal on innocent children”, when it struck a children’s cancer hospital in Kyiv earlier this week.

Sir Keir said Britain would provide £3 billion per year to Ukraine and speed up the delivery of military aid.

As the “generational threat” from Russia demands a “generational response”, he confirmed plans to boost British defence spending to 2. 5 per cent of gross per cent of their GDP on defence,” he said. “But in light of the grave threats to our security, we must go further.”

Earlier this year Lord Cameron, the former foreign secretary, met president Zelensky in Kyiv, where he made comments about Ukraine’s right to defend itself which were interpreted by Russia as a “dangerous” threat to use British-gifted missiles on Russian territory.

Adml Sir Tony Radakin, the Chief of the Defence Staff, later clarified that the cruise missile could only be used inside Crimea and the mainland of Ukraine.

Jamie Shea, a former Nato official, on Wednesday responded to Sir Keir’s statements on the Storm Shadow as giving Mr Zelensky a “shot in the arm”.

The Ukrainians “have to be able to strike back against those significant military targets”, he said.

But on Thursday, Mr Zelensky appeared to recognise that full permission for cross-border strikes had not been granted.

“We have got very good messages from the UK leader,” he told a Nato press conference, but said no decision had been made yet.

Meanwhile, Britain’s strategic defence review will be launched next week, setting out how Sir Keir intends to meet his spending target.

Closer defence ties

Announcing the review, John Healey, the Defence Secretary, insisted that seeking closer defence ties with Brussels would not bind the UK into a European defence force.

The Defence Secretary said, however, that Britain would seek to join more EU military programmes.

He suggested that Britain could seek to join more areas of the EU’s permanent structured co-operation (Pesco) programme.

The UK is already part of one Pesco scheme on moving military equipment across the EU.

Mr Healey said: “So we’ve proved we can do it. There are one or two other programmes which may well be programmes in which we have a unique contribution to make and we can get a great deal out of being part of a programme like that.

“That does not require any new formal agreement, it does not require a EU-UK security pact in order to do that.”

Mr Healey acknowledged that talks on the formal security pact with the EU sought by the Government were unlikely to start this year.