LONDON -- UK's Northern Ireland Secretary James Brokenshire resigned Monday, citing health grounds for his unexpected decision.

His decision to quit will be seen as a major blow for Prime Minister Theresa May as she embarked on a much trumpeted reshuffle of her team of ministers.

Brokenshire, a key May supporter, had not been mentioned in any of the speculative pre-shuffle reports and had been expected to stay in his cabinet job.

He was seen as a crucial minister at Stormont where the devolved Northern Ireland assembly is currently suspended following a fall-out between the Democratic Unionist Party (DUP) and the pro-republican Sinn Fein.

The assembly has not met since last March, with Brokenshire involved in trying to broker peace between the two sides to avert a return of direct rule from Westminster over Northern Ireland.

The Daily Telegraph cited a source close to Brokenshire saying that he was set to undergo major surgery within the next few weeks for a lesion to his right lung.