A tourist was locked inside a London book shop for two hours after it shut while he was still browsing.

David Willis, from Texas in the United States, went downstairs in Waterstones in Trafalgar Square on Thursday night to find all the lights out and the door locked.

Police were called to the shop after he described his plight on Twitter, and someone with a key finally turned up to free him just after 11pm.

Mr Willis posted a picture of himself behind shutters with the message: "This is me locked inside a Waterstones bookstore in London.

"I was upstairs for 15 minutes and came down to all the lights out and door locked. Been here over an hour now. Supposedly someone is on their way."

In a later post which was retweeted by more than 7,000 people, he wrote: "Hi Waterstones, I've been locked inside of your Trafalgar Square bookstore for two hours now. Please let me out."

When he was finally released, he wrote: "I'm free."

A Scotland Yard spokesman said police were called at around 9.35pm "by a man claiming to be locked inside a shop".

He added: "Officers attended and following the arrival of a key holder the man left the building shortly after 11pm."

The Waterstones' website says the Trafalgar Square store closes at 9pm.

The book retailer posted a message on its Twitter feed saying: "We're pleased to announce that [Mr Willis] is a free man once more. Thanks for your concern and tweets."