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 each carry a maximum penalty of five to 10 years in prison.

In exchange, prosecutors agreed to drop 13 additional counts for crimes including wire fraud and making false statements.

Prosecutors and Rocha’s lawyer indicated they had agreed a sentence but details were not disclosed in court on Thursday.

He is due back in court on April 12 when he is likely to be sentenced.

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

Rocha was arrested by the FBI at his Miami home in December on allegations that he engaged in “clandestine activity” on Cuba’s behalf since at least 1981 — the year he joined the US foreign service — including by meeting 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 at the State Department for two decades at posts in Argentina, Bolivia, Mexico and the US Interests Section in Havana.

He followed that with a lucrative post-government career that included a stint as a special adviser to the commander of the US Southern Command.

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.

In those recordings 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.

Rocha’s decision to plead guilty came hours after the widow of a prominent Cuban dissident killed in a mysterious car crash filed a wrongful death lawsuit against the former diplomat.

The lawsuit accuses him of sharing intelligence that emboldened Cuba’s communist leaders to assassinate a chief opponent.