"China's authorities have under certain circumstances detained foreigners citing 'endangering national security'," the British Foreign Office said in its latest advice posted on its website.
"There is also a risk of arbitrary detention, including of British Nationals," it said.
British advice previously contained no reference to the risk of arbitrary detention.
A spokesman for the British embassy in Beijing said the advice had been updated to "clearly and factually reflect recent incidents."
"But the level of our advice has not changed," the spokesman said.
The British warning comes after several foreigners were detained in China on national security charges, including Canadians, Australians, Japanese and at least one American. Some of them remain in detention.
In one of the latest high-profile cases, Cheng Lei, an Australian citizen and anchor on Chinese state television, was detained in August for "carrying out criminal activities endangering China's national security," according to China's foreign ministry.Her detention follows a period of strained relations between China and Australia.