Kate Middleton, Duchess of Cambridge, has announced the due date of her second child with husband Prince William.

Middleton, 33, was on a royal visit to London's Brookhill Children's Centre on Tuesday, said British press, when she responded to a maternity query from one of its volunteers.

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"I told her she is beautiful and that she's got a tiny little bump," Christie Osborne told reporters after the visit.

"I asked when her baby is due and she said mid-to-end of April."

Middleton and William, 32, already are parents to Prince George, who turns 2 in July.