A former US ambassador has said he will admit serving as a secret agent for communist Cuba going back decades, bringing an unexpectedly fast resolution to a case prosecutors described as one of the most brazen betrayals in the history of the US foreign service.

Manuel Rocha, 73, told a federal judge he would admit to federal counts of conspiring to act as an agent of a foreign government, charges that could land him behind bars for several years.

His defence lawyer indicated that prosecutors have agreed a sentence, but the length was not disclosed in court on Thursday.

Rocha is due back in court April 12.

“I am in agreement,” Rocha said when asked by US District Court Judge Beth Bloom if he wished to change his plea to guilty.

Prosecutors alleged that Rocha engaged in “clandestine activity” on Cuba’s behalf since at least 1981 — the year he joined the US foreign service — including by meeting with Cuban intelligence operatives and providing false information to US government officials about his contacts.

Federal authorities have said little about what Rocha did to assist Cuba while working for the State Department and in a lucrative post-government career that included a stint as a special adviser to the commander of US Southern Command.

Rocha, whose two-decade career as a US diplomat included top posts in Bolivia, Argentina and the US Interests Section in Havana, was arrested by the FBI at his Miami home in December.

The case relies largely on what prosecutors say were Rocha’s own admissions, made over the past year to an undercover FBI agent posing as a Cuban intelligence operative named Miguel.

Rocha praised late Cuban leader Fidel Castro as “Comandante”, branded the US the “enemy” and bragged about his service for more than 40 years as a Cuban mole in the heart of US foreign policy circles, the complaint says.

“What we have done… it’s enormous… more than a Grand Slam,” he was quoted as saying in one of several secretly recorded conversations.