Forty-one-year-old Neil Heywood was found dead in a hotel room in Chongqing, China's biggest metropolis, last November. His death was initially attributed to alcohol poisoning but foul play is now suspected.
On April 10, Chinese authorities made the unexpected announcement that Gu Kailai -- the wife of the region's former Communist Party chief, Bo Xilai -- was being investigated on suspicion of murder, along with a family aide, Zhang Xiaojun.
Hague said in a written statement that he welcomes the Chinese commitment to probe the death fully.
"We now wish to see the conclusion of a full investigation that observes due process, is free from political interference, exposes the truth behind this tragic case, and ensures that justice is done," he said.
Prime Minister David Cameron met visiting Chinese official Li Changchun in London Tuesday afternoon, and Hague was scheduled to meet him later.
Conspiracy theories, political skullduggery and poisoning have blossomed as the scandal unfolds around Heywood's death and Bo's fall from grace.
Bo was suspended from the Communist party's Central Committee on the same day the murder investigation into his wife was announced. "Comrade Bo Xilai is suspected of being involved in serious disciplinary violations," said the Chinese news agency Xinhua.
Hague said Foreign Office officials were first made aware on January 18 of rumors within China's British expat community of possible suspicious circumstances in Heywood's death.
Allegations about Heywood's death were then made by former Chongqing vice-mayor and chief of police Wang Lijun during a visit to the U.S. Consulate in Chengdu on February 6, Hague said.
The foreign secretary said he was informed of the claims the next day and immediately instructed British officials to ask the Chinese authorities to investigate.
In mid-February, a senior British diplomat met Chinese counterparts, Hague said. "He informed them of our concerns about Mr. Heywood's death and the suspicion that he had been murdered, and conveyed our formal request that the Chinese authorities investigate."
That request was conveyed to Chinese officials twice more, once in February and again in March, Hague said. Announcing the investigation on April 10, the Chinese assured Britain's envoy to Beijing that "proper judicial process" would be followed, he said.
Taking questions from lawmakers in the Commons Tuesday, Hague rejected a suggestion that the Foreign Office had been too slow to act.
"We are pursuing this extremely carefully but vigorously," he said, adding that British officials are supporting Heywood's family.
A commentary piece published by Xinhua on Monday said the case was being "handled according to Party regulation and discipline, reflecting the Party's resolution to strictly govern itself. It does not indicate a political struggle within the Party."
