LONDON (AP) – Jeremy Heywood, a powerful government adviser who ran Britain’s civil service until illness forced him to step down, has died of cancer, the government said Sunday. He was 56.

Prime Minister Theresa May said Heywood, who died early Sunday, “worked tirelessly to serve our country.” She called his death “a huge loss to British public life.”

As Cabinet secretary since 2012, Heywood oversaw Britain’s government bureaucracy and was responsible of putting ministers’ plans into action. That made him an influential though little known figure outside the Whitehall government district.

He had previously been principal private secretary to Labour Prime Minister Tony Blair, chief of staff to Blair’s successor Gordon Brown and Downing St. permanent secretary under Conservative Prime Minister David Cameron.

Brown said Heywood “was a unique civil servant who may not always have agreed with proposals from ministers but always had a positive and often better alternative to offer.”

Liberal Democrat leader Vince Cable said Heywood had advised politicians of right and left, and “none of us had the faintest idea what his politics were. He was just the perfect civil servant.”