Chinese police have ordered the Chinese widow of British businessman Neil Heywood not to speak to foreign media.

It was announced in state media this week that Gu Kailai, the wife of the politician Bo Xilai, is in custody suspected of the 41-year-old's murder. Bo has been suspended from his posts and is under investigation for "serious disciplinary violations".

Relatives of Heywood had said that they believed his death in Chongqing last November was due to natural causes. But Tuesday's announcement said Gu, 52, and a staff member at the family home had been detained and were "highly suspected" of intentionally killing the Briton. State media said there had been a conflict over "economic interests".

Heywood's widow Wang Lulu, a Chinese citizen, has not commented publicly on the case. "Police officers questioned her recently and warned her not to speak to foreign media," a source with direct knowledge of the case, who wished to remain anonymous, told Reuters.

Another source close to the family said: "It's still difficult to believe. It was absolutely not necessary. The two families were very close. She [Gu] was the godmother of [Heywood's] children." There were UK media reports that Wang had visited the British embassy to ask for a visa so she and her two children could escape from China.

In an indication of the government's anxiety to control discussion of the biggest upheaval in Chinese politics for years, mobile phone users in Beijing have found that they cannot send or receive text messages using the Chinese characters for Bo Xilai or Gu Kailai. People in other cities said they could include the names without problems when sending messages locally, but texts they sent to Beijing users did not arrive. While internet censorship is extensive in China, restrictions on text messaging are much less so.

Bo, 62, had been seen as a contender for elevation to China's top political body when a new generation of leaders takes power later this year. Neither he nor his wife have been seen since he was dismissed as party secretary of Chongqing last month.

Some believe that a commentary on tackling corruption, published in the official party newspaper, People's Daily, hinted at allegations against the couple. The piece accused corrupt officials of receiving bribes or other payments via spouses, children, relatives and friends or even mistresses. Some committed corruption overseas, transferred embezzled money or assets there, or secretly obtained foreign citizenship, it added.