As a result of the dispute with Heywood, Gu had become convinced Heywood was a threat to her son, Bo Guagua, the official said without elaborating.
"Gu Kailai believed that Neil Heywood had threatened the personal safety of her son Bo (Guagua) and decided to kill him," the official added, reading from a statement to a packed news conference of dozens of reporters who had been barred entry to the courtroom in the eastern city of Hefei.
The aide, Zhang, had driven Heywood to Chongqing last November from Beijing and prepared a poison which was to be put later into a drink of water. Later that day, Heywood met Gu at a hotel, he became drunk and then asked for water.
"She poured a poison into his mouth," the official said.
Gu and Zhang face the death penalty if convicted. But many legal experts expect Gu will be convicted but only sentenced to a lengthy jail term, citing her desire to protect her son, who graduated from Harvard this year, as a mitigating factor.
Gu's state-appointed lawyer told the court on Thursday that Heywood himself had some "responsibility in the matter," the court official said, adding that a Heywood family representative had voiced respect for the court during the hearing.
In London, family members declined to comment on the case.
Britain's Foreign Office also declined to comment until the outcome of the case. It said two British diplomats had attended the trial "to observe the proceedings and fulfill consular responsibilities to the Heywood family," a spokesman said.