Money from a peace-keeping fund used to support fragile and war-torn states has been included for the first time in the official measure of the UK's defence spending, helping keep it comfortably above the target of 2% of GDP set for Nato member states.
The £1 billion-a-year Conflict Pool delivers conflict prevention, security, justice and capacity-building programmes in fragile states, and is jointly funded by the Ministry of Defence, Foreign Office and Department for International Development.
New Nato figures have shown that the UK is one of just five of the organisation's 28 member states to meet the target of spending 2% of GDP on defence this year.
The 2.1% spend in 2015 was down from 2.2% last year and 2.3% in 2013. The average between 2005 and 2009 was 2.4% and in the late 1990s 2.5%.
Ministers have come under intense pressure to commit the UK to the 2% level for future years, but Defence Secretary Michael Fallon insisted on Sunday that future plans will not be disclosed until Chancellor George Osborne's cross-government spending review in the autumn.
Defence sources confirmed that the MoD's £450 million contribution to the Conflict Pool was being included in the Nato calculation for the first time.
A spokesman said: "As with other Nato member states, from time to time the UK makes updates to ensure defence spending is categorised fully in accordance with Nato guidelines and what it says should be included."
Other Nato states hitting the 2% target for 2015 were the United States (3.6%), Greece (2.4%), Poland (2.2%) and Estonia (2%).
Senior Conservative backbencher Bernard Jenkin said: "If this is money that's given to the MoD and spent by the MoD on armed forces activities and military diplomacy, then that would be alright.
"If it is just re-badging money that is spent on other things, then it's not alright."