Shortly before David Cameron met a senior Chinese Communist party official in Downing Street, Hague highlighted government impatience with the Chinese by revealing that it took the authorities two months to act on British concerns.
The government first raised its concerns with Beijing on 15 February. The authorities announced their investigation on 10 April.
In a written statement to MPs, Hague said Jeremy Browne, the Foreign Office minister, was not told of Heywood's death despite visiting Chonqqing on 15 and 16 November.
Heywood was found dead in a Chongqing hotel room on 15 November. Browne met Bo Xilai, the discredited former party chief in the city, on the morning of 16 November.
The foreign secretary said: "I welcome the fact that the Chinese authorities have now committed themselves to undertake the investigation into Mr Heywood's death that we sought. 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.
"We will continue to engage with the Chinese authorities on the progress of the investigation and we stand ready to provide any assistance necessary. Foreign and Commonwealth Office [FCO] officials will remain in close touch with Mr Heywood's family as this investigation proceeds."
Cameron met Li Changchun, a senior member of the politburo standing committee of the Communist party of China. The prime minister's spokeswoman said before the meeting: "The prime minister will take the opportunity to raise the case of Neil Heywood, welcoming the launch of the Chinese investigation and emphasising that we are keen to address concerns about the suspicious circumstances of his tragic death. We want to see the conclusion of an investigation that observes due process."
Hague's statement gave a detailed chronological account of the events that followed Heywood's death. He said:
• British consular officials in Chongqing were notified of Heywood's death on 16 November by fax from the public security bureau of the Chongqing municipality. "Chinese officials informed our staff that the cause of his death was overconsumption of alcohol."
• Heywood's family informed the Foreign Office on 18 November that they wished to have his body cremated. A Foreign Office official attended the ceremony.
• Foreign Office officials were made aware about suspicions surrounding the official explanation of the cause of death from 18 January.
• Wang Lijun, the former Chongqing vice-mayor and chief of police, made allegations about Heywood's death when he visited the US consulate in Chengdu on 6 February.
• Hague was informed about concerns surrounding the case on 7 February. He asked officials to make urgent representations to the Chinese authorities.
• The deputy head of the British embassy in Beijing met officials from the Chinese foreign ministry on 15 February to raise Britain's concerns. "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."
• The British ambassador made the same request on 21 February to the Chinese ministry of foreign affairs vice-minister for Europe. The ambassador repeated the request a week later to the director general for Europe.
• In the absence of a formal response, the Foreign Office's consular director raised the case with a visiting Chinese official on 22 March.
• The Chinese authorities announced an investigation on 10 April.
