Michelle Obama took her daughters to meet the stars of Harry Pottter in Leavesden yesterday and enjoyed a party in Hogwarts Grand Hall, the Observer can reveal.

The trip to the South Way studios was an eighth birthday treat for the Obamas' youngest daughter Sasha.

A source close to the Leavesden studios, said the First Family arrived on site at about 1pm and were taken on a tour of the set before celebrating with author J K Rowling and a large group of youngsters.

Young Sasha and sister Malia, 10, both avid Harry Potter fans, were then treated to a surprise visit by star Daniel Radcliffe. The source said Mrs Obama, who had her hair tied back in a bun, looked ‘very elegant’.

Leavesden resident Stephen Liddell was making his way to St Albans when he spotted a police motorcade leaving the old aerodrome field at 4.15pm.

Mr Liddell, of Haines Way, said: “There was a large black range rover, another black security car with lights flashing and police motorcyclists in front and back. I had just crossed the bridge over the A41 near the top of Leavesden High Road.

“I followed the road system round to go on the dual carriageway towards the cinema and to my surprise they passed right in front of us and went off towards St Albans too.”

The Obamas have been exploring London this week after travelling to Europe for the D-Day anniversary commemorations with President Barack Obama.