UK and NATO allies agree to boost spending on defence and related areas to 5% of GDP by 2035

The UK and its NATO allies have agreed to increase spending on defence and related areas to 5% of GDP by 2035, two diplomatic sources with knowledge of the deal have told Sky News.

Ambassadors of all 32 alliance member states signed off on the new spending pledge ahead of a major summit of leaders, including Donald Trump, this week, they said. However, Spain's prime minister later signalled that his country - which has been resisting the push by allies to expand its already underperforming defence budget to such a level - did not need to comply.

This could become a point of friction with the US president who has repeatedly warned that he would not defend a member state that is not in his eyes paying its fair share. The boost - up from a current goal of 2% of GDP - is as much about keeping the US president onside as it is about responding to what is regarded by the allies as a growing threat from Vladimir Putin and the challenge posed by China.

The target will be formally rubber stamped when the heads of state and government meet in The Hague on Tuesday and Wednesday. The ambitious spending goal - secured following a huge amount of persuasion by Mark Rutte, the NATO secretary general - is broken down into 3.5% of GDP spent on pure defence and 1.5% of GDP spence on related areas, which can include infrastructure and cyber security.

Spain remains a potential problem for alliance unity though. Pedro Sanchez, the Spanish prime minister, last week said he would not commit to the 5% figure.

It prompted efforts to find a compromise in the wording of the agreement that will be endorsed at the summit, according to the Reuters news agency. It reported that the language around the spending pledge had been tweaked from "we commit" to "allies commit.