Democratic Unionist deputy leader Nigel Dodds has been taken to hospital in London after falling ill at Westminster, senior party sources have said.
The North Belfast MP was in the party's offices at the House of Commons when the episode unfolded just before Prime Minister's questions.
He was taken to a nearby hospital.
It is understood Mr Dodds, 54, is undergoing a series of tests.
Mr Dodds, a former finance minister in the Northern Ireland Executive, is married to sitting DUP MEP Diane Dodds.
A Cambridge graduate, he has been a Member of Parliament for North Belfast since 2001.