Kate Middleton may have blurted out a royal secret - the newest addition to the royal family could be a baby girl.

Kate, 31, is pregnant with her first child and the baby, the first to Prince William and the Duchess, will be third in line to the throne behind Prince Charles and its father.

And if Kate's faux pas is to be believed, the royal family will celebrate the birth of a future queen, in the summer.

The Duchess, who was on an official visit to Grimsby, received a teddy bear from a fan and, according to a reportin The Sun, Kate Middleton said: "Thank you, I will take that for my d... for my baby."

A third woman, identified as Sandra Cook, who was standing nearby, was quoted as saying: "As she handed it over I distinctly heard Kate say, 'Thank you, I will take that for my d...' Then she stopped and corrected herself to say, '... for my baby'."

Cook says she then questioned the Duchess about the slip, to which Kate Middleton politely replied: "We're not telling."

"It was worth every cold minute. This lady giving her a teddy seemed to take her by surprise. Everyone else was giving flowers. I'm certain it's a girl. She wouldn't have said the D-word otherwise. After she said it, she was a little flustered," Cook concluded.

Kate's royal visit to Grimsby was a solo one, as Prince William was busy with RAF responsibilities.

The Duchess' little remark was made during a visit to the National Fishing Heritage Centre. She was also present for the official opening of the Havelock Academy and visited a local hospice.

The visit to Lincolnshire should have been a relaxing one for the Duchess, following a holiday in the Caribbean that was interrupted by Chi magazine's publication of photographs capturing her in a bikini, with her baby bump.

Britain's Catherine, Duchess of Cambridge arrives at Havelock Academy in Grimsby, in northern England March 5, 2013.Reuters
Britain's Catherine, Duchess of Cambridge speaks to children during a visit to the Havelock Academy in Grimsby, northern England March 5, 2013.Reuters
Britain's Catherine, Duchess of Cambridge poses for a photograph with Princes Trust volunteers during a visit to Peak Lane fire station in Grimsby, northern England March 5, 2013.Reuters
Britain's Catherine, Duchess of Cambridge speaks with former trawler-man John Vincent during her tour of the National Fishing Heritage Centre in Grimsby, northern England March 5, 2013.Reuters
Britain's Catherine, Duchess of Cambridge sits next to a dummy of the Ship's Mate on the Skipper's Table, on display at the National Fishing Heritage Centre in Grimsby, northern England March 5, 2013.Reuters
Britain's Catherine, Duchess of Cambridge receives flowers from children during a visit to Peak Lane fire station in Grimsby, northern England March 5, 2013.Reuters
Britain's Catherine, Duchess of Cambridge watches a performance by students during a visit to the Havelock Academy in Grimsby, northern England March 5, 2013.Reuters