LONDON (Reuters) - Britain's Prince William and his wife Kate are expecting a baby in July, the prince's office announced on Monday, ending speculation that the Duchess of Cambridge may be having twins.

Press and public interest in their first baby will be huge, with particular focus on gender now that the government is passing legislation changing historic rules of succession to ensure the baby will become third in line to the British throne behind William no matter whether it is a girl or boy.

"Their Royal Highnesses The Duke and Duchess of Cambridge are delighted to confirm they are expecting a baby in July," the prince's office said in a statement.

The pregnancy of the 31-year-old, still popularly known as Kate Middleton, was revealed in December when she was admitted to hospital for four days suffering from a type of severe morning sickness which is slightly more common among women expecting twins.

"The Duchess's condition continues to improve since her stay in hospital last month," the statement added.

The duchess was forced to cancel several public engagements due to the illness. She was last seen in public on Friday when she visited the National Portrait Gallery in London with William for the unveiling of her first official portrait.

William, 30, the eldest son of heir to the throne Prince Charles, married Middleton in April 2011 and the newlyweds were given the titles Duke and Duchess of Cambridge.

Their sumptuous royal wedding was watched by a global audience estimated at up to two billion people, turning them into A-list celebrities whose every step is followed by the world's media.

William, a helicopter rescue pilot and grandson of Queen Elizabeth, is second in line to the throne, meaning his first child will become third in succession when he or she is born. Previously, male heirs took precedence over females in rules of succession.

There was a predictable flood of congratulations from members of the public via social networking sites, with some people suggesting a public holiday of the kind announced to mark the queen's Diamond Jubilee last June.

Spoof Twitter accounts belonging to fictional members of the royal family also seized on the news.

"One can confirm that William and Kate's baby is growing faster than the economy," wrote a "Prince Charles" on Twitter, who has more than 300,000 followers despite his "fictional" status.