Image copyright PA UKIP leadership hopeful Steven Woolfe has been taken ill at the European Parliament, a party spokesman says.
The MEP was taken to hospital in Strasbourg for tests.
Mr Woolfe announced on Wednesday he will stand as a candidate for the party's leadership after Diane James stepped down.
The other candidate to have declared so far, Raheem Kassam, tweeted his best wishes for Mr Woolfe, as did ex deputy chairwoman Suzanne Evans.
Mr Woolfe, who celebrates his 49th birthday today, is said to have been taken ill after walking out of a vote at the parliament in Strasbourg.
Mr Kassam tweeted that he had cancelled his appearance on BBC2's Daily Politics "out of respect" for Mr Woolfe, following reports he had been taken ill.
He added: "I really hope @Steven-Woolfe is okay. Plz send him your best wishes."
Ms Evans, who is said to be considering her own leadership bid, tweeted: "Shocked to hear Steven Woolfe has apparently collapsed in the European Parliament in Strasbourg. Wishing him well for a speedy recovery."