Paramedics were called to the House of Lords to attend a medical emergency outside the main chamber on Tuesday evening.
It is understood that a peer collapsed in the Prince’s Chamber, an anteroom next to where members debate.
The patient was treated at the scene, before being taken to hospital.
A London Ambulance Service spokesperson told the PA news agency: “We were called at 6.38pm today to reports of an incident at Palace Of Westminster, SW1P.
“We sent a number of resources to the scene, including an ambulance crew, an advanced paramedic, an incident response officer, a medic in a fast response car, a clinical team manager and a paramedic on a motorcycle.
“We treated a patient at the scene before taking them to hospital as a priority.”
The House of Lords press office said: “We can confirm that medical staff are attending an incident in Prince’s Chamber.”
Despite the emergency, proceedings continued inside the Lords main chamber as peers discussed the crisis in Israel and Gaza.
