Thousands of well-wishers are lining the gates of Buckingham Palace, eagerly anticipating official news of the royal baby's arrival.

Under a scorching London sun, hordes of tourists and locals are waiting patiently following a morning announcement the Duchess of Cambridge was in labour at St Mary's Hospital across town.

Hoping to catch a glimpse of the easel, due to be placed on the forecourt detailing confirmation of the birth, many had already secured their spot along the Palace fence with cameras and smart phones at the ready.

Making their presence felt directly in front of the main gates were sisters Sue and Pauline, on holiday from New Zealand and Australia respectively, clad in their national flags.

While the primary reason for their visit to London was to see family members, they didn't want to miss a chance to be part of "historic occasion".

"My sister always thought it was going to be born on Monday so we were ready to come down," Sue said.

"We're happy to stay here and we're hoping to see the easel. Hopefully it's not after midnight, though.

"I don't think it can be too far away now. She went in at 6am didn't she?

"We'd like it to be a girl because there will be two kings after the Queen, so it'd be nice by then to have a Queen again."

Joining the ensemble of visitors were journalists, camera crews and, as always at the Palace, a strong police and security presence.

The level of excitement was matched by confusion among some.

"I'm going to ask them about the easel, I can't see it," one observer said to his friend.

"It's not up yet. The baby hasn't been born!," the friend responded.

And clearly not everyone present had caught royal baby fever.

"The baby?" one woman said in a thick eastern European accent when asked whether she too had come to welcome the third in line to the throne.