DUP MP Nigel Dodds is to stay in hospital overnight after taking ill while in London.
However, the party is hoping he will be released on Thursday morning.
The 54-year-old north Belfast MP took ill in the party office in London on Wednesday morning.
It is understood he complained to staff of having a “severe headache”.
He is currently undergoing tests to discover if he had suffered a problem with his heart.
The DUP deputy leader was in Westminster to attend the House of Commons for Prime Minister’s Questions (PMQs).
However, he was missing from the benches as he had taken ill shortly before PMQs at mid-day.
His wife and DUP MEP Diane Dodds, who was in London at the same time on business, is now at his hospital bedside.
DUP leader Peter Robinson MLA said on Wednesday night: “Earlier today whilst preparing for Prime Minister’s Question Time, in our Westminster party office, Nigel felt unwell.
“An ambulance was called and he was taken to a local hospital where he has received medical treatment.
“I have spoken with Diane this evening, Nigel is in good spirits and is feeling much better. He will be staying in hospital over night for observation but hopes to be discharged in the morning.
“Everyone in the party wishes him a full and speedy recovery.
“Nigel and Diane would appreciate some privacy and have no plans to make any further comment at this time.”
Many people have used social networking sites to pass on their best wishes to Mr Dodds and his family.
PUP leader Billy Hutchinson tweeted: “Wishing Nigel a speedy recovery.”
Former party colleague and bar library friend Jim Allister also send his best wishes on Wednesday night.
The TUV leader said: “Wishing Nigel Dodds a speedy and full recovery.”
Last week, Mr Dodds joined party colleagues Peter Robinson MLA, Jeffrey Donaldson MP and Gregory Campbell MP to attend the funeral of Baroness Thatcher at St Paul’s Cathedral.