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"A major ramp-up" in children's mental health services is needed to cope with the problems faced by young people, the boss of NHS England has said.

Speaking to The House magazine, Simon Stevens said social media was contributing to higher rates of mental health problems in young people.

He also said more people were "willing to come forward" as the stigma around mental health reduces.

Theresa May has pledged to give the NHS an extra £20bn a year by 2023.

Mr Stevens, the chief executive of the NHS in England, said expanding mental health services for young people was "one of the big things" that had to come out of the long-term NHS plan.

He suggested that survey results, yet to be published, would show a big increase in young people experiencing mental health problems - a rise he attributed partly to the internet.

"There is a growing awareness that alongside some of the positive aspects of children's online experience and social media, there are some important negatives," he said.

He also identified gaming addiction as a problem and said social media companies and schools should help search for solutions.

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During the interview Mr Stevens commented on the debate over whether cannabis could be used for medical purposes.

Earlier this month 12-year-old Billy Caldwell, who suffers from severe epilepsy, was permitted to take cannabis oil which reduces his seizures.

Mr Stevens said there was "some evidence" that compounds derived from cannabis could help certain health conditions such as multiple sclerosis.

However he stressed that the issue should be separated from the debate around legalising cannabis for recreational use.

His comments come as a report from the Institute of Economic Affairs said legalising cannabis would raise £1bn a year in tax for the UK.

The report has valued the UK's black market in cannabis at £2.6bn.