The couple were arrested at their residence in Yangon and have been sent to the notorious Insein prison, according to local reports.
A brief statement from a foreign affairs spokesperson in London said authorities were “concerned by the arrest of a British woman” and providing consular assistance.
The arrest is likely to deepen the isolation of Myanmar’s military junta, known as the Tatmadaw, which seized power in a coup last year. The military has cracked down on dissent, arresting thousands of protesters and political figures, including Nobel Peace Prize laureate Aung San Suu Kyi.
Human Rights Watch called for the couple’s release, saying the arrest was “outrageous and unacceptable.”
“Once again, the Tatmadaw show why they are among the worst of the worst rights abusers in Southeast Asia,” deputy Asia director Phil Robertson tweeted.
On Thursday, Britain imposed sanctions on businesses linked to the military, although the announcement appeared unrelated to the arrests. Britain is also backing a genocide case against Myanmar at the International Court of Justice for its treatment of Rohingya Muslims, and has expressed support for Myanmar’s National Unity Government, a shadow administration operating in exile.
Britain was among many countries to condemn the junta’s recent execution of pro-democracy activists. Its top diplomat in the country, Pete Vowles, said last month he was “forced by the junta to leave but glad we didn’t cave to pressure to legitimize their brutal coup.”
Cabato reported from Manila, and Tan, from Singapore.