Newly unearthed top-secret documents reveal that the secret service of communist-era Czechoslovakia spied on US President-elect Donald Trump and his first wife, Ivana, in the 1970s and 1980s. One informant noted in 1977 that Trump "is completely tax-exempt for the next 30 years".

Czechoslovakia's intelligence agency, the Státní bezpečnost or StB, spied on the real estate mogul and his Czechoslovak-born wife when she made frequent trips back to her home country to visit her father, Miloš Zelníček. According to The Guardian, the files were stamped "top secret" and bore the code names "Slusovice," "America" and "Capital".

The secret files were discovered by Czech television and the German tabloid Bild. The StB dossiers include files dedicated to the couple's pre-nuptial agreement in which Trump agreed to pay his wife $1m (£800,000) if their marriage ended. The couple ended their relationship in 1992.

An informant going under the cover name "Lubos" reported in 1977 that Ivana had started working at a petrol station in Austria, where she had met her first husband, Alfred Winklmayr. She then emigrated to Canada, where she would eventually marry the future president.

A fellow spy reported in 1977 that Trump's businesses had received commissions from the state, making them "absolutely safe". The spy added: "Another advantage is the personal relationship [he has] with the American president [presumably Jimmy Carter] and the fact that he is completely tax-exempt for the next 30 years."

Another informant under the cover name "Milos" reported in 1988 that Trump was under significant pressure to make a run for the White House. Milos added that Ivana was also under pressure not to make a wrong move during her visits to Czechoslovakia so as to not risk her husband's chances at a successful candidate.

"Any false step of hers will have incalculable consequences for the position of her husband who intends to run for president in 1996," the informant wrote. Milos added that Trump was confident he could win. The assessment, however, came 20 years too early.

The Guardian reported that the StB also sent a spy to the US to monitor Trump. The belief was that if he became US president it could have a significant impact on relations between the two countries. StB spy "Al Jarda" noted that on 10 October 1989 a delegation from the communist agricultural production cooperative from Slusovice, the village where Ivana's father live, visited Trump.

"They were given a welcome by one of the richest men in New York, Mister Donald Trump. He got them to explain to him extensively about the work of the cooperative and its further plans in the field of trade," Al Jarda wrote. Trump was reportedly invited to visit Slusovice.

StB's archives reveal that phone calls between Ivana and her father were tapped at least once a year and their mail constantly monitored. Ivana's father was registered as an "StB confidant" though a Czech historian said that it did not mean he worked as an agent. "Rather the CSSR authorities forced him to talk to them because of his journeys to the US to see his daughter," Tomáš Vilímek told Bild. "If he hadn't spoken to them he would not have been given permission to fly."

The StB dissolved in 1990 after the collapse of communism.