Tobias Ellwood, the Conservative foreign minister, was praised as an "absolute hero" after he tried to save the life of a policeman stabbed in the Parliament attack.
The Tory MP attempted mouth-to-mouth resuscitation and applied pressure on the wounds to stem the blood flow to keep him alive but said he had "lost too much blood".
Ellwood, a former soldier himself, attended the victim as he awaited medical staff and an air ambulance to arrive in Parliament Square. He was later seen with bloodied hands talking to police officers near the scene before going back to the Foreign Office.
The police later confirmed the officer died. He was stabbed when he confronted a man who had just mowed down pedestrians on Westminster Bridge and ran into the grounds of parliament.
Ellwood said: "I am a witness, I was the last one on the scene before he died so I am not allowed to say anything. Number 10 has been in touch so I'll get into trouble.
"It is now murder, I was on the scene and as soon as I realised what was going on I headed towards it. It is a huge tragedy, it really is.
"I tried to stem the flow of blood and give mouth to mouth while waiting for the medics to arrive but I think he had lost too much blood.
"He had multiple wounds, under the arm and in the back.
"As I understand it the attacker has also died. I'm not allowed to say anything, I'll get into trouble. It's very sad."
A source close to Elwood told The Telegraph: "[He] tried to give mouth to mouth and stem blood flow from multiple stab wounds to the officer until the chopper and medics arrived".
There were reports that Ellwood "ran in [the] opposite direction to everyone else, he ran towards the injured police officer".
Photographs later emerged of Ellwood, dressed in a blue suit, helping medical staff and policemen treat the officer, who later died.
A source close to the MP said last night he was "well but shaken".
Elwood, MP for Bournemouth East, has been Parliamentary Under-Secretary at the Foreign Office since July 2014, covering the Middle East and Africa brief.
He lost his brother Jon in the Bali terror attack in October 2002 which killed 202 people including 27 Britons.
MPs from across the political spectrum praised him as a "hero". Sir Alan Duncan, a Tory minister who works with Ellwood in the Foreign Office, told The Telegraph: "He has done very well. He was the right man in the right place."
Sir Alan was among MPs being held in the House of Commons chamber when the officer's death was announced. He said: "In the Chamber there was a real somber mood when they announced the policeman had died. And equally there is massive admiration for Tobias Ellwood.
"Good for him. He has a military background and he is the sort of person who leaps into action. He is just a proper get-on-with-it guy in a crisis. I'm going to give him a very stiff drink."
Ben Howlett, the Tory MP, tweeted: "Tobias Ellwood is an absolute hero for what he did to help the policeman this afternoon!"
Tim Farron, the Liberal Democrat leader, said: "Today, Tobias gave MPs a good name. He was utterly heroic, pure and simple. He went above and beyond and did all he could to save a police officer."
Jake Berry, the Tory MP for Rossendale and Darwen, said: "What an amazing and brave action from this former soldier. "He showed compassion for everyone who works here in Parliament and every police officer who works to keep us safe.
Ellwood has previously spoken about how hard it was to get his brother's body home after being killed in the Bali bombing.
"We just wanted to bring my brother's body back to Britain, as simple as that. But it's not that simple," Ellwood said in 2012. "You need death certificates in both languages, embalming certificates, sealing certificates.
"All these processes require time and people to participate. None of that happened. I had to do many of those things myself to the point where I ended up screwing the lid down on the coffin myself. That can't be right."