The British businessman whose murder has sparked political upheaval in China was poisoned, two sources with knowledge of the police investigation have said.

They say the businessman threatened to expose a plan by a Chinese leader's wife to move money abroad.

It was the first time a specific motive has been revealed for Neil Heywood's murder last November.

The death has ended Chinese leader Bo Xilai's hopes of emerging as a top central leader and has thrown off balance the Communist Party's looming leadership succession.

Bo's wife, Gu Kailai, asked Heywood late last year to move a large sum of money abroad, and she became outraged when he demanded a larger cut of the money than she had expected due to the size of the transaction, the sources said.

She accused him of being greedy and hatched a plan to kill him after he said he could expose her dealings, one of the sources said, summarising the police case. Both sources have spoken to investigators in Chongqing, the south-western Chinese city where Heywood was killed and where Bo had cast himself as a crime-fighting Communist Party leader.

Gu Kailai is in police custody on suspicion of committing or arranging Heywood's murder, though no details of the motive or the crime itself have been publicly released, other than a general comment from Chinese state media that he was killed after a financial dispute.