Rene Gonzalez is one of the so-called Cuban Five, a group of agents sent to the US by Cuba to infiltrate Cuban exile groups Havana says were plotting attacks against the Castro government
The five were arrested in Miami in 1998 and found guilty of infiltrating US military bases and Cuban exile groups, and passing the information to Cuba.
Mr Gonzalez was jailed for 15 years. Havana says the Cuban Five are political prisoners and is lobbying the Obama administration for their release.
However, the US Supreme Court ruled last year that it will not consider the case, ending their hopes of an appeal.
Mr Gonzalez spoke to the BBC in a rare interview from his prison cell in Marianna, Florida.
Here are excerpts of what he told the BBC's Matt Frei.
I came into contact with some of the organisations in Miami who had been doing violent activities for years against the Cuban government. I went to Brothers to the Rescue (a US-based organisation of Cubans opposed to Fidel Castro's government) and another one ... organisations with beautiful names but bent on committing violent activities against Cuba My role was to inform the Cuban government what they were doing.
We don't like to use the term spying because that term is easy to be manipulated. When it comes to the law, spying is to go after government secrets ... I would say that I was working undercover with some criminal organizations.
It is unfair to keep someone in prison for fighting terrorism. I have been informing to my government on terrorist activities and all of a sudden I find myself facing a 15 year sentence! It would be crazy if it wasn't so politically charged.
Wife denied visa to visit
My wife hasn't seen me for eight years and Gerardo's wife hasn't seen him for 12 and that is one of the most cruel endeavours that the US government has taken against us.
The men are considered national heroes in Cuba
I don't know, maybe they're trying to break our marriages in order to break our spirits. It is very cruel. The last time I saw her was my birthday in 2000, and for Gerardo, the last time was 1997 or 1998.
She's applied about eight times ... she's been denied eight times. The reasons vary but they basically say that it is a national security threat."
Prison life then and now
Right after our arrest, we were in solitary confinement it was very rough treatment back then. It was part of the whole setup in order to try to break us down. That lasted for 17 months, but that's part of the past and I suppose that's how they view their jobs in regard to softening us up for the trial. Then we went to the regular facilities and so far I can't complain about that.
I do a lot of exercising. I have a job in the recreation yard. When I finish I exercise. I run. Then I go back to my cell and do a lot of reading. I took a course in Economics from Havana University. I feel I am making the best of my time here.
I treat everyone with fairness and I receive the same treatment. Some prisoners are more politically aware than others - they approach me and ask me questions and try to understand my point.
Obama and US-Cuba relations
There was a time when I had some hopes, because of the way he spoke and how he handled himself. But the way I see it from here, he has been a little bit hypersensitive to the right wing of this country. And right now I don't see any improvement.
Posters of the five can be seen in Havana
They are asking the Cuban government to give concessions that we cannot give we don't impose any conditions on your government to improve relations. What we want is to have a normal relationship by respecting everybody's system. We don't try to topple their government, we don't apply pressure for regime change in the US. We have our own form of government and they should respect that.
My generation grew up witnessing years of aggression coming from the US - we're talking about terrorism, bombings, shootings - so my generation understands very well that we have the right to defend Cuba from all those crimes. So it is not only the Cuban government but the whole society that understands our cause.
And nobody in Cuba would agree to any re-establishing of normal relations with a country that has five of their sons in jail for defending the country.
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