Britain’s stretched army is increasingly relying on 16- and 17-year-olds to fill its ranks, the youngsters accounting for nearly 30% of those enlisted last year, the highest proportion since the start of the decade.

Recruits are also more likely to come from poorer backgrounds, such as on the edges of cities in the north of England, according to data from the Child Rights International Network (CRIN), which campaigns against recruiting under-18s.

Charlotte Cooper, campaigns coordinator with CRIN, said: “The army is leaning on teenagers from the most deprived backgrounds to fix its recruitment crisis, using them to fill the riskiest roles because it can’t persuade enough adults to enlist.”

Britain is the only Nato alliance member and country in Europe to allow direct enlistment into the army at the age of 16, a longstanding but controversial practice that has been repeatedly defended as a vital source of recruitment.

Youngsters are not eligible for combat operations until the age of 18, but the period is used for training at the Army Foundation College, in Harrogate. Parental consent is also required, although only in the form of a signature.

GCSE results day, which falls this Thursday, is often the target for recruiters. Recruitment offices in cities such as Liverpool can stay open until 7pm. Official figures have shown the army spending heavily on Facebook advertising in previous years.

The most recent official recruitment figures released show that in the year to the end of March 2019 the army enlisted 1,000 16-year-olds and a further 820 17-year-olds, accounting for 28.8% of recruitment into the ranks. The army also recruited more 16-year-olds than any other age, the figures show.

The figures make up the highest proportion of youth recruitment since 2010/11, and partly reflects the fact that enlistment of the young is holding up better while overall recruitment is plunging. Last year only 6,320 people were enlisted into the ranks of the British army, the lowest level since modern records began in 1999/2000.

Lloyd Russell-Moyle, a Labour MP, said it was hardly surprising the army was so dependent on under-18s. “The government has privatised soldiers’ accommodation and recruitment, kept pay desperately low, extended leave, and over-programmed and underfunded all three armed forces.

“Two decades of needless political wars have also not helped. If you make military service that unpleasant you will have to rely on applicants’ youth and naivety.”

Other figures obtained by the Guardian this month showed that frontline army units were as much as 40% under strength. Nine of the army’s 16 infantry regiments were at least 20% below strength.

CRIN also said that the army targeted poorer parts of the UK in its youth recruitment, and according to its analysis of enlistment data relating to England, the rate of recruitment of the under-18s is 57% higher in the most deprived fifth of constituencies relative to the most affluent fifth.

Blackpool South, Hereford (where the SAS is based), and Hull East, were the three places where, between 2013 and 2018, the most 16- and 17-year-olds signed up. By contrast no youngsters were recruited during that period from a string of London constituencies, including Chipping Barnet, Ealing Central, Hackney South, and Tooting.

Quick guide Minimum age for military recruitment around the world Show Hide The UN’s optional protocol to the convention on the rights of the child (Opac) on the involvement of children in armed conflict aims to protect children from recruitment and use in hostilities. It entered into force in February 2002 and commits signatories to not recruiting children under the age of 18 or sending them to battle, or conscripting soldiers under 18. According to the last annual report by the charity Child Soldiers International, 109 countries have a minimum age of 18 for enlistment as well as deployment. However, 46 state militaries around the world continue to recruit children under the age of 18 into their armed forces.

Countries still recruiting soldiers at the age of 16 include Bangladesh, Egypt, El Salvador, Iran, Mauritania, Pakistan, Papua New Guinea, the UAE, the UK and Zambia.

Among EU nations, at 16, the UK has the youngest recruits. Most EU states only recruit at 18, although Austria, France and the Netherlands recruit at 17, and in Belgium you can join the army early if you have completed your education.

Some states, such as Eritrea, Somalia, Sudan, South Sudan and Syria, have a recruitment age legally set at 18, but in practice recruit younger children into conflict. The US, China, Australia, Canada and New Zealand allow recruitment at 17. Russia has a minimum enlistment age of 18.

CRIN csaid it had been forced to do the analysis because the Ministry of Defence did not “collect information on the socioeconomic profile of its recruits”. But it added that there was plenty of further information to back up its suggestion that poorer youngsters were a target.

The brief for the army recruitment campaigns in 2018 and 2019 specified the target audience as 16- to 24-year-olds from the lower socioeconomic categories of C2DE. The brief for 2017’s This Is Belonging campaign was geared to young people from families with a mean income of £10,000.

The Ministry of Defence said that the percentage of under-18 recruits to the army fluctuated year-on-year, and that at 28.7% last year was far lower than the peak level of 44% reached between the attacks in the US on 11 September 2001, and the Iraq war.

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An MoD spokesperson added: “We are proud of the opportunities serving in the armed forces affords young people, from basic literacy education and support for postgraduate degrees, to high-quality accredited training and unique employment prospects.”

Officials also denied that the army’s recruitment campaign aimed to be directed at those from deprived backgrounds around the country, saying its advertising was “designed to reach a broad range of audiences”.