The Conservative MP, face bloodied, gave first aid to PC Keith Palmer but could not stop him from dying from his stab wounds outside of the Palace of Westminster.
“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,” Mr Ellwood told The Sun.
Two pedestrians were also killed and 29 were hospitalised in the London terror attack; the number of walking wounded is unknown.
MPs from across the political spectrum have commended Mr Ellwood for his bravery.
Labour leader Jeremy Corbyn said: “Our thanks and gratitude go to the police and emergency services who responded so bravely, and to those – including the MP Tobias Ellwood – who went to the aid of the injured and dying.”
Conservative MP Ben Howlett tweeted: “Tobias Ellwood is an absolute hero for what he did to help the policeman this afternoon!”
Liberal Democrat leader Tim Farron said: “Today, Tobias gave MPs a good name.”
London terror attack: Tobias Ellwood performed mouth-to-mouth on PC Keith Palmer
London terror attack: Sadly, PC Palmer died of his injuries
London terror attack: Mr Ellwood had blood on his face form his brave efforts
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Who is Tobias Ellwood?
Mr Ellwood, 51, is the MP for Bournemouth East and a former Captain with the Royal Green Jackets. In 2002, his brother Jonathan was one of 202 who died in the Bali terror attack.
Today, Tobias gave MPs a good name
Jonathan, a teacher, was in the country for a conference and was killed when a bomb tore through the clubbing district of Kuta.
Mr Ellwood and his family did not find out that Jonathan was dead until three days after the tragedy, after which the MP flew out to Bali to bring his brother’s body home.
Speaking after the atrocity, Mr Elwood said: “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.”
London Terror Attack: Latest Pictures Thu, March 23, 2017
4 dead and 40 injured in Westminster terror attack
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Conservative MP Tobias Ellwood (centre) helps emergency services attend to a police officer outside the Palace of Westminster, London
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He was born in New York to British parents and grew up in Germany and Austria before returning to the UK to study at Loughborough University.
In 1991 Mr Ellwood joined the Army. Over six years he served in Northern Ireland, Cyprus, Kuwait, Germany, Gibraltar and Bosnia.
On leaving the Army, he entered politics as a researcher for the former Defence Secretary Tom King.
Mr Ellwood works alongside Boris Johnson
Mr Ellwood meeting Tobias Ellwood meeting Kuwaiti Foreign Minister Sheikh Sabah Khaled al-Sabah
He later returned to education, studying for a master’s degree at the Cass Business School at City University.
After graduating Mr Ellwood took up a post as a Senior Business Development Manager at the London Stock Exchange, which he followed with a similar position at a law firm.
In 2005 Mr Ellwood was elected to Parliament. In July 2014 he was appointed parliamentary under-secretary at the Foreign Office, with special responsibility for the Middle East and Africa.
Mr Ellwood and his wife Hannah have two young sons, Alexander and Oscar. Their constituency home is in Strouden, Bournemouth, and Mr Ellwood is a governor at the local Queen’s Park Infant School.