The Duke and Duchess of Cambridge are expecting their first child, they have announced.
Regardless of whether the baby is male or female, the child will succeed the throne after Prince William.
Reaction has been quick to emerge from public figures and royal-watchers.
Prime Minister David Cameron was quick to offer his congratulations. He used Twitter to say: "I'm delighted by the news that the Duke and Duchess of Cambridge are expecting a baby. They will make wonderful parents."
Mr Cameron later told BBC News that he had been "tipped off" about the news shortly before it was made public: "A little note came into the meeting I was having and I found it quite difficult to keep it to myself."
Labour leader Ed Miliband - who was alerted to the news by BBC chief political correspondent Norman Smith - tweeted: "Fantastic news for Kate, William and the country. A royal baby is something the whole nation will celebrate."
Deputy Prime Minister Nick Clegg and his wife later hailed the "brilliant, brilliant news".
Mr Clegg said: "And by happy coincidence, we're literally working right now in government to put the finishing touches to legislation which will update the very old-fashioned rules of succession which mean that if they have a baby girl, regardless of whether the baby girl then has younger brothers in the future, she will be able to succeed to the throne."
Scotland's First Minister Alex Salmond released the following statement: "My warmest congratulations and sincere best wishes to the Earl and Countess of Strathearn on this wonderful news. Everyone in Scotland will join me in wishing the couple the very best as they prepare for the birth of their first child."
Wales First Minister Carwyn Jones wrote on Twitter: "Many congratulations to the Duke and Duchess of Cambridge on their wonderful news."
St James's Palace broke the news in a statement which said: "The Queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall and Prince Harry and members of both families are delighted with the news."
William's uncle Earl Spencer welcomed the announcement, saying in a statement: "It is wonderful news and I am thrilled for them both." The baby would have been a first grandchild for William's late mother and the Earl's sister, Diana, Princess of Wales.
Meanwhile the Archbishop of Canterbury, Rowan Williams, who married the royal couple in Westminster Abbey in April 2011, said: "The whole nation will want to join in celebrating this wonderful news. We wish the Duchess the best of health and happiness in the months ahead."
President of the United States, Barack Obama, congratulated the royal couple via a White House spokesman: "On behalf of everyone here in the White House, beginning with the president and first lady, we extend our congratulations to the Duke and Duchess of Cambridge on the welcome news this morning out of London that they are expecting their first child."
Australia's Prime Minister Julia Gillard offered her "congratulations to Prince William and Kate, adding: "This is delightful news. It's going to bring joy to them and to their family, and I think it's going to bring joy to many around the world.
"Clearly it is a time of joy and it can also be a time of challenge. And I'm sure many will be thinking of Kate as she deals with morning sickness and is in hospital. But from the Australian people to Prince William and Kate, delightful news and our congratulations."
New Zealand's Prime Minister John Key said: "Great news for William and Kate, and obviously for the royal family - they'll be very excited.
"Obviously we wish them very, very well. And clearly Kate is going through a bit of morning sickness so there'll be a lot of New Zealand women that will be able to sympathise with that. We wish her a speedy and healthy... back to fitness and health."
Canada's foreign minister John Baird addressed parliament, saying: "Mr Speaker, I would be remiss if I first didn't stand up and extend our congratulations to the Duke and Duchess of Cambridge on the announcement that came out earlier today."
Around the UK
The University of St Andrews, where the then Kate Middleton and Prince William met, released a statement saying the royal couple "must be very pleased".
A spokeswoman added: "We are delighted for the couple and will be writing to them to offer our congratulations."
The Royal Air Force (RAF), with which William serves as a search-and-rescue pilot, said: "The RAF is delighted with the news and wishes the Duke and Duchess of Cambridge all the best for the future."
The Football Association sent its "best wishes to our president, HRH the Duke of Cambridge and his wife, the Duchess".
Local councillor Graham Pask, in the duchess's home village of Bucklebury in West Berkshire, said the news "had made his evening" and he was "truly delighted".
The news was likely to turn the media's attention once again on to the village, but Mr Pask said most locals did not have a problem with it: "If a few reporters rock up it's fine by me, it puts us on the map a bit. Life still goes on. She will always be a Bucklebury lass as far as we are concerned."
Journalists and commentators
Royal journalist Ingrid Seward said: "Everybody loves a royal wedding and everyone loves a royal baby, and so it's wonderful news for all of us media. It's also a very happy occasion for the duke and duchess."
Ms Seward said the royal pair are likely to have wanted to start a family "sooner rather than later", adding: "Kate will be 31 in January and I think by royal standards that is relatively old. [Princess] Diana was pregnant with Prince William very, very quickly, as was Princess Margaret."
Graham Smith of anti-monarchy group Republic said the news was a "private, personal matter" for the duke and duchess, adding that the tide of media coverage was disproportionate.
"We've heard today that our future head of state is on the way. It's a pretty bizarre way of choosing someone for public office," he said.
Royal-watcher Penny Junor said the duchess's pregnancy was "very, very good news and the news everyone has been waiting for".
She added: "I imagine they will be treated with a little more privacy... William is a highly private man. I think that the press, particularly in the wake of the Leveson Inquiry, will be a little more restrained."
Royal historian Kate Williams told the BBC: "The royal popularity is really soaring high after some pretty steady lows over the past 50 years or so."
She added reaction to the duke and duchess' firstborn was likely to be unprecedented: "The bells rang across Britain when Queen Victoria had her son, the future Edward VII. But it's nothing like this. I think William and Kate are now going to be besieged by babygros, toys and tables and all kinds of books.
"This is a global phenomenon and is perhaps the most famous child in modern history."
Global journalists and commentators
The royal baby-to-be means unalloyed happiness for Prime Minister David Cameron, wrestling as he was with the Leveson fall-out and welfare reform and the euro-crisis, but now likely to be revelling in the spotlight turning to what an Independent columnist has sourly dubbed "the feel-good foetus".
Reporters are staking out a hospital, news websites have launched live-blogs and a "forensic artist" has even engineered images of a yet to be born child... The British tabloid websites celebrated in typical fashion.
The waiting game is on. On the heels of a not-as-planned pregnancy announcement, the Royal Family finds itself in a tense period of waiting for the Duchess of Cambridge to recover from a troublesome and rare condition of acute morning sickness. Prince William was back at his wife's side in hospital Tuesday while speculation continued that the Royal Family may be in line for more than one new baby.
The news should help everyone forget the previous big news about the duchess this year: the embarrassing publication of a series of topless - and one or two bottomless - photographs taken illicitly while she and the duke were on vacation in France... It gives Britain something to be excited about at a time when life here has not been so exciting, what with austerity and widespread flooding across huge parts of England after a period of nearly biblical rainfall.
The Duchess of Cambridge will be a natural mom, according to pals close to Prince William's wife of 18 months... The child - boy or girl - will jump ahead of William's kid brother, Harry, in line for the throne now that the UK enacted a constitutional change that gives a firstborn girl the right to succeed the throne over her younger brother.