Veteran British war correspondent Clare Hollingworth, who broke the story that the Second World War had started, turned 100 in Hong Kong on Monday as a memoir of her is near completion.

The centenarian was joined by about 150 journalists and friends at a party at the Hong Kong Foreign Correspondents' Club (FCC), where she has been a regular in the past three decades since moving to the city in 1981.

"Yes, I am happy but I don't want any sensation," the frail bespectacled woman said when asked how she felt about turning 100.

"I hope I behave," she joked, as congratulatory messages from all over the world poured in, including from veteran BBC correspondent John Simpson, who described her as "magnificent".

British foreign secretary William Hague said Hollingworth was "one of the UK's leading foreign and war correspondents", and congratulated her on an "extraordinary career".

"You truly had a front seat at history," Hague said in a message read out by the British consul-general Andrew Seaton in Hong Kong.

The veteran journalist, who witnessed the horrors of war in Vietnam, Algeria, the Middle East, India and Pakistan, is best remembered for her scoop on World War II in 1939, when she was just a rookie reporter.

She broke the story of Germany's invasion of Poland during her first week working as a journalist in Poland after she was dispatched there by The Daily Telegraph to cover the worsening security situation in Europe.

On September 1, Hollingworth called the British Embassy in Warsaw to tell the officials that the war had started, after she had been woken by the roar of Nazi aircraft and tanks.

Hollingworth has said she had to hold the telephone out of her bedroom window to capture the sounds of German forces in order to convince the embassy that her story was true.

"If there is a war, and if the world wants, I would like to cover it," the journalist, still proud of her scoop, told AFP in a 2009 interview.

Today Hollingworth lives with two helpers in Hong Kong. Her memory has deteriorated badly after a suspected stroke in 2009, said her great nephew Patrick Garrett, who is writing Hollingworth's memoir.

"She is surprisingly strong physically, though she looks frail," Garrett told AFP, adding that he aims to publish the memoir by next year.