Hereford MP Paul Keetch is still in a London hospital after falling ill on a flight to the US.
Paul Keetch MP was born and bred in Hereford
The Liberal Democrat was on his way to the Nato Assembly in Washington DC on Sunday when he suffered chest pains.
The plane was forced to turn round and bring him back to Heathrow Airport. His condition on Tuesday was described as stable.
Lib Dem president Simon Hughes, who is also a close friend of Mr Keetch, said he was shocked to hear the news.
Mr Hughes said: "I was very worried and surprised.
"Paul is a generally fit and healthy individual. I don't think he has had any serious illness in his life and certainly not in the time I've known him since he was a teenager."
A spokeswoman for the Liberal Democrats said she could not confirm what was wrong with Mr Keetch, but said he had undergone tests on Monday.
Mr Keetch, who was born and bred in Hereford, had announced he is stepping down at the next election after being elected in 1997.
The MP, who is married with one son, has also been on the city's council since 1983.