Kim Kardashian may not be the only one expecting a baby girl this summer, as the Duchess of Cambridge may have hinted to the fact that her firstborn may be a girl too.

The news is intended to be a royal secret but Kate Middleton may have inadvertently slipped up about the gender of the baby she and Prince William are expecting.

On Tuesday, only her third day of official engagements this year, she visited the National Fishing Heritage in Grimsy and walked among the 2,000-person crowd who had waited up to six hours in freezing fog for her arrival, The Daily Telegraph reported.

Diana Burton, a local, handed Middleton a white teddy bear, which the Duchess accepted with a smile.

"Thank you, I'll take that for my d-" Middleton said before she broke off her comments.

Sandra Cook, who was standing near Middleton, told reporters she asked the royal if she was about to say "daughter" and was told at first that the royal couple did not know the child's gender, and then that the couple would not reveal if the baby was a boy or girl.

"I said to her: 'You were going to say daughter weren't you?' and she said, 'No, we don't know.'" Cook continued, "I said, 'Oh, I think you do' to which she said: 'We're not telling.' "

Middleton is due to give birth in July. At the event she also spoke with another admirer, Bobbie Brown, revealing that the royal baby has been very active.

"I asked her if [the baby] has been moving or kicking," Brown said. "She said: 'Yes, it is. Very much so.'"

Middleton's baby bump was covered by a brown Hobbs coat and a Great Plains dress. She later visited a fire station in Grimsby, where the invited guests included patients from a local hospice.

Claire Moss-Smith, 86, told her: "I'm waiting for you to be Queen."

Middleton replied: "You might be waiting a long time!"