The Duchess of Cambridge is in the early stages of labour and has been admitted to St Mary's Hospital in London. Kensington Palace issued the waiting media throng with a short statement on Monday.
Catherine travelled by car from the palace to the hospital in Paddington, west London, with her husband Prince William, the Duke of Cambridge on Monday morning (local time). The couple do not know the sex of their first child, who will be third in line to the throne.
There is likely to be no more news until the official announcement of the birth, the BBC reports. The world's press have been camped outside St Mary's for days in anticipation of the birth. The due date has never been officially announced, but has widely been expected to be mid-July.
Royal vehicles were seen at a back entrance to the hospital about 6am on Monday, with the announcement coming from Kensington Palace 90 minutes later.
The duchess is being tended by a top medical team led by the Queen's former gynaecologist Marcus Setchell, who delivered the Countess of Wessex's and Prince Edward's two children.
The Duke of Cambridge has been with his wife on annual leave and will have two weeks' paternity leave.
The way the birth will be announced is steeped in tradition. It is custom for news of royal births and deaths to be attached to the railings of Buckingham Palace but in this case, it will be displayed on an ornate easel in the forecourt of the palace, the BBC reports.
The Queen, senior members of the Royal Family, and the duchess's family - if they are not at the hospital - will be told about the birth first.
A royal aide will then take the bulletin, signed by key medical staff, from the hospital to the palace under police escort. After the note is displayed, an announcement will be posted on Twitter and Facebook, and the media will be informed.