Boris Johnson has promised a further £55 billion in defence spending over the rest of the decade in response to the threat posed by Vladimir Putin’s Russia.

The Prime Minister committed Britain to increasing defence spending to 2.5% of gross domestic product (GDP), a measure of the size of the economy, by the end of the decade and encouraged Nato allies to boost their own military budgets.

Mr Johnson pledged the rise on Thursday after public lobbying from Cabinet ministers Ben Wallace and Liz Truss.

It is understood the pledge could amount to an extra £55.1 billion cumulatively over the rest of the decade in order to reach the 2.5% goal in 2030, based on Office for Budget Responsibility forecasts of the size of the economy.

Speaking as the Nato summit in Madrid drew to a close, Mr Johnson said: “We need to invest for the long term in vital capabilities like future combat air whilst simultaneously adapting to a more dangerous and more competitive world.

“The logical conclusion of the investments on which we propose to embark, these decisions, is that we’ll reach 2.5% of GDP on defence by the end of the decade.”

Defence Secretary Mr Wallace, who was at the centre of a Cabinet spending row over his high-profile campaign for more cash, was said to be grateful for the announcement.

A source close to the minister said: “The Defence Secretary has always been clear that, as the threat changes, so should defence spending.

“In 2020 the Prime Minister reversed decades of under-investment in defence and he rightly responded to Russia’s danger by continuing to invest in defence, for which the Defence Secretary is very grateful.”

But Tobias Ellwood, the Tory chairman of the Commons Defence Committee, said it is “too little, too late”.

Mr Ellwood, who has called for 3% of GDP to be spent on defence, also condemned Mr Johnson for going ahead with planned cuts to the size of the Army.

“This is NOT the time to cut the Army by 10,000,” he said on Twitter.

“And moving to 2.5% defence spend by 2030 is too little too late.”

Conservative MP Julian Lewis, the chair of Parliament’s Intelligence and Security Committee, described the spending increase as “feeble”, accusing the Prime Minister of “an inability or unwillingness to face up to the gravity of the current crisis”.

Mr Johnson said the West needed to show Mr Putin it had the “staying power” to support the Ukrainians for the duration of the conflict.

“If you wanted evidence of the amazing ability of the Ukrainians to fight back, to overcome adversity, and to repel the Russians, then look at what has happened just today on Snake Island where Russia has to cede ground,” he said.

The UK has promised an extra £1 billion of military aid for Ukraine and Mr Johnson said Volodymyr Zelensky’s country had to be supported to take back occupied territory.

“I think if Ukraine were to be crushed or forced into a bad peace, the consequences for freedom around the world would be appalling,” he said.

Ukraine has lobbied for Nato membership, something which would have guaranteed protection from the alliance when Mr Putin launched his invasion in February.

Mr Johnson said membership of the alliance – which is set to expand to include Finland and Sweden due to decisions made in Madrid – was an argument for “down the track”.

But as an interim position he said Ukraine should be fortified with “Nato-grade weaponry, plus intelligence, plus training” so that a future attack was inconceivable.

The gathering in Madrid also saw Nato take a firmer view on China, saying it challenges Nato’s “interests, security and values”.

Mr Johnson said: “It’s very important that we remember that we have a huge economic relationship with China … but at the same time, we’ve got to understand that there are areas where we need to compete, contest and sometimes challenge what China is up to.

“There has to be a balanced approach and every country around that table can see that, but obviously one of the reasons why what is happening in Ukraine is so important is because there is a clear read-across to other theatres, and that’s why we’re sticking up for the rules-based international system in the way that we are.”

Foreign Secretary Ms Truss has argued that the Russian invasion of Ukraine shows the need to ensure Taiwan has the arms it needs to deter a Chinese invasion.