Prime Minister David Cameron has led the congratulations to the Duke and Duchess of Cambridge after the news that they are expecting their first child.
In a Twitter message issued minutes after St James's Palace announced the news of the pregnancy, Mr Cameron said: "I'm delighted by the news that the Duke and Duchess of Cambridge are expecting a baby. They will make wonderful parents."
Labour leader Ed Miliband said: "This is wonderful news for the Duke and Duchess of Cambridge. The whole country will be thrilled. Justine and I offer Kate and William our warmest congratulations."
Scottish First Minister Alex Salmond also offered his congratulations to the couple, who are known as the Earl and Countess of Strathearn when in Scotland.
Mr Salmond said: "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."
A spokeswoman for the University of St Andrews, where Kate and William met, said: "We are delighted for the couple and will be writing to them to offer our congratulations. They must be very pleased."
In a later comment to television cameras at 10 Downing Street, Mr Cameron confirmed he was informed of the news shortly before the public announcement. He said: "I got a little note as I came into a meeting I was having and I found it quite difficult to keep it to myself."
Deputy Prime Minister Nick Clegg and his wife hailed the "brilliant, brilliant news". He said: "Miriam and I are as happy and excited as I can imagine the rest of the country is at the news that the Duke and Duchess of Cambridge are going to have a baby.
"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. So it's a very exciting day, but also we're making sure that if it is a baby girl, she can be our queen."
Jay Carney, press secretary to US President Barack Obama, said: "On behalf of everyone here in the White House, beginning with the President and the 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."