The acclaimed journalist David Hencke, whose career at the Guardian spanned more than three decades, has died of liver cancer aged 79.

As Westminster correspondent, Hencke was instrumental in exposing the cash-for-questions scandal that forced the resignations of two Conservative ministers, and the scoop that led to Peter Mandelson’s first resignation from government.

The Guardian’s editor-in-chief, Katharine Viner, said: “David Hencke was a true Fleet Street legend. He worked for the Guardian for more than 30 years and was responsible for breaking some of the biggest political stories of the time.

“David became the most feared journalist in Westminster because of his acute nose for political scandal and wrongdoing. He worked with an enthusiasm and energy that inspired colleagues and rivals over an impressively long and important career.”

Francis Beckett, a distinguished journalist who worked with Hencke on three books, said: “David discovered early in life something he loved doing and was very good at. And he was a lucky man; he was able to do it for all of his working life. And what he loved was finding things out that rich and powerful people didn’t want us to know, and telling us.

“Working with him on the three books we did together, I saw regularly the excitement it gave him to find something that was genuinely new, that somebody powerful had tried to hide, and put it in the book.”

Beckett recalled how Hencke’s relaxed demeanour and keen nose for a story made him a formidable scoop getter. “He looked and sounded completely harmless. If I had been a politician with a secret and I had looked at David, I can perfectly well imagine I would have confided in him.”

Hencke was, he revealed, still working on a story until a week before his death on Friday. “That was what he loved doing.”

Hencke joined the paper as a reporter in 1976 before graduating to his role as Westminster correspondent, a position he held until his departure in 2009. He went on to work as an investigative journalist.

Hencke was named reporter of the year in 1994 for his coverage of the cash-for-questions scandal. The story was key in raising the public’s awareness of Tory sleaze in the 1990s, a prominent issue at the 1997 general election that ended 18 years of Conservative rule.

It eventually led to the resignations of the ministers Neil Hamilton and Tim Smith. The latter stepped down as an MP, while the former was defeated by the journalist Martin Bell, who stood on an anti-corruption platform.

Hencke also won scoop of the year in 1998 for revealing Peter Mandelson’s secret £373,000 home loan with his beleaguered government colleague Geoffrey Robinson. The cash, Hencke and his colleagues reported, enabled Mandelson to buy his £475,000 Notting Hill home.