Trial ends within eight hours, with Gu's lawyer claiming Briton Neil Heywood bore some responsibility for his own murder

The British businessman Neil Heywood bears "a certain responsibility" for his own death, a lawyer defending Gu Kailai has told her murder trial, as the first official account of how she is alleged to have murdered the Briton emerged.

Prosecutors said Gu – the wife of the disgraced Chinese politician Bo Xilai – drank with Heywood in his hotel room until he vomited, then, when he wanted water, administered the poison that killed him.

The hearing into China's greatest cause celebre for decades was dispatched in a few hours on Thursday. Gu and the family employee Zhang Xiaojun did not contest the allegation of premeditated murder, said Tang Yigan, vice-president and spokesman of the Hefei intermediate people's court. "Gu Kailai is the main culprit and Zhang is the accomplice," he added.

The judgment will be delivered at an unspecified future date, Tang said in his brief statement to reporters after the hearing closed, which included the details of the case.

Bo, once tipped for promotion in the 10-yearly change of party leadership approaching this autumn, was ousted as party secretary of Chongqing in March. His downfall, and the extraordinary news of his wife's arrest on suspicion of murder, was the biggest upheaval in Chinese politics for 20 years.

Gu's trial is so sensitive it was not even mentioned on China's flagship evening news programme. But in a short clip carried on the state broadcaster's website, Gu was shown arriving at the courtroom wearing a black suit and white shirt, flanked by two female police officers.

Zhang, her co-accused, wore a white polo shirt and dark trousers as he walked between two male officers.

The clip said four Chongqing police officers had been charged last week with covering up the murder of Heywood by Gu to prevent her from having to face criminal responsibility. They will stand trial in the same court in Hefei on Friday.

The official statement released on Thursday did not explain why Gu's lawyer, Jiang Min, had said the 41-year-old Briton should bear responsibility for his murder. But officials said Gu had killed him in the belief that he posed a threat to her son and alluded to a conflict over money.

Prosecutors alleged that Gu arranged for Zhang to escort Heywood from Beijing to south-western Chongqing on 13 November last year. She met him in his room, number 1605 at the Lucky Holiday hotel, where they drank tea and alcohol together until he was so drunk that he vomited and needed water.

She then poured poison that she had prepared earlier, and had been carried by Zhang, into Heywood's mouth.

The 41-year-old's family were told he died of excessive drinking. They accepted the conclusion, apparently believing it brought on a heart attack. But friends of the businessman were sceptical, saying he was only a light drinker.

Intriguingly, Gu's defence lawyer said she had alerted officials to crimes of other people and that this should be considered a significant contribution and taken into account. No further details were given.

When news of her detention first emerged, some thought Bo might also be tried. But no further mention has been made of his case since the initial statement that he was under investigation by party authorities for disciplinary violations.

Some also surmised that the decision not to charge Gu with economic crimes, to which she allegedly confessed, was an attempt to shield her husband – and by extension other leaders and their families – from unwelcome scrutiny over possible corruption.

Defending Gu, Jiang said Gu's ability to control her behaviour was "weaker than normal people's" at the time of Heywood's death. But the court noted that she was in good physical condition and emotionally stable during Thursday's hearing.

No foreign media were allowed into Gu and Zhang's trial, with officials saying there was no room in the spacious courtroom, although empty seats could be seen in the news footage.

Two British diplomats attended in a consular capacity. Relatives and friends of Heywood and the defendants were also present, the statement added.

Dozens of plain-clothed officers and scores of uniformed police surrounded the courthouse during the hearing. Two singing protesters who appeared outside the Hefei intermediate people's court in Anhui were dragged away in the pouring rain. "I don't believe it. This case was decided well in advance," said one of them, Hu Jiye, before he was pushed into a car.

The lawyer Li Fangping wrote on Sina's Weibo microblog: "The security of the Hefei intermediate court is definitely number one in the world but whether it can ensure just proceedings is another question."

Responding to another lawyer's post, the artist and activist Ai Weiwei wrote on Twitter: "The 'justice' of no justice, no fairness, and no openness to the public. It is sad."

Relatives of Gu, 53, and Zhang, 33, hired lawyers to represent them, but were then told that the defendants had accepted state-appointed lawyers. But Gu thanked the judge, prosecution and her own lawyers at the end of proceedings and asked that Zhang receive a lighter sentence than hers, a source who witnessed the trial told the Washington Post.

"I committed a crime that brought negative consequences to the party and the country," she added, according to the source.