The Duchess of Cambridge received a “Baby on board” badge during a surprise visit to Baker Street Underground station with the Queen and Duke of Edinburgh on Wednesday morning.

Ham & High: The Duchess of Cambridge arrives at Baker Street Underground station for an event celebrating the 150th anniversary of the Tube. Picture: Nick Ansell/PAThe Duchess of Cambridge arrives at Baker Street Underground station for an event celebrating the 150th anniversary of the Tube. Picture: Nick Ansell/PA (Image: PA Wire/Press Association Images)

The royal trio turned up at the Tube station to celebrate the 150th anniversary of the transport network – the Queen’s first public event in more than a week after falling ill.

She chatted to senior London Underground managers, train drivers and station staff during her visit.

As the royal party stood on a platform, London Underground chief operating officer Howard Collins presented the iconic white badge to the Duchess, to the amusement of watching guests.

She joked “I’ll make sure I wear it at home”.

Mr Collins said afterwards: “She asked me, ‘How do they work?’ and I said they’re fantastic, they really do make a difference – saves men the embarrassment of having to guess if a woman is pregnant.

“She asked so many great questions of the staff and seemed really interested in our work.”

Meanwhile just half a mile away, the Duke of Edinburgh was due to open the £3.6million Tiger Territory at London Zoo.

The brand new Sumatran tiger enclosure is home to Jae Jae and Melati, Europe’s most genetically important pair of Sumatran tigers who were matched by the global breeding programme for the critically-endangered species, which number just 300 in the wild.