Harry Potter star Emma Watson was stopped at US immigration – because officials thought she was a child travelling alone.

The 22-year-old was singled out after landing at New York’s JFK airport and told she should be accompanied by an adult, The Sunday People has revealed.

The flustered actress – who played Hermione Granger in the Harry Potter films – was eventually waved through and managed to laugh off the incident.

But she later revealed she is often mistaken for a kid – and blamed the fact she was wearing a schoolgirl-style backpack.

A source said: “Emma was mortified. Not only did they not recognise her, they thought she was a child. It was very embarrassing but she managed to awkwardly laugh it off.

“The Americans are very serious about who they let in and out of their country. If young kids are thought to be travelling on their own it poses a massive problem.

“Luckily Emma managed to convince them how old she was.”

The British actress, who recently finished filming her latest movie This Is The End, later told of her embarrassment on Twitter.

She wrote: “Passport control: ‘Unaccompanied minor?’ Me: ‘Sorry?’ Passport control: ‘Where is ur guardian?’ Me: ‘I’m 22!!!!!!!’ #neverwearingabackpackagain.”

She added: “The really sad thing is that this is not the first time this has happened .”

The star was returning to the US where she was expected to continue her studies at Brown University, Rhode Island.

She has had two years out from her degree to work on film projects including Noah, starring Russell Crowe and Anthony Hopkins, which is due out next year.

She was flying in from Zambia, where she had been working for British charity Camfed on a mentoring programme she has been involved in since the summer. The source said: “Emma’s a big fan of Camfed and is incredibly proud to fly their flag for them.”