DUP MP Nigel Dodds is being treated in hospital after taking ill while attending the House of Commons at Westminster.
Mr Dodds, who is the MP for north Belfast, is said to be in a stable condition in hospital.
He was in the party's offices when he became unwell just before prime minister's questions this morning.
He was taken to a nearby hospital.
It is understood the 54-year-old 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.
In a statement, DUP leader Peter Robinson said Mr Dodds will be staying in hospital overnight for observation but hopes to be discharged in the morning.
"Everyone in the party wishes him a full and speedy recovery," he said.