Labour MP Jo Cox, who was earlier today attacked in Birstall near Leeds, has died in hospital.
The MPs behind both Vote Leave and Stronger In have suspended EU referendum campaigning.
Cox was shot and stabbed outside a library in Birstall where she holds her weekly advice surgery. She was then airlifted to Leeds General Infirmary.
A man in his 70s also reportedly suffered slight injuries.
A 52-year-old man has been arrested, West Yorkshire police said in a statement. He has been named locally as Tommy Mair.
Jeremy Corbyn MP, leader of the Labour Party said:
The whole of the Labour Party and Labour family - and indeed the whole country - will be in shock at the horrific murder of Jo Cox today.
In the coming days, there will be questions to answer about how and why she died. But for now all our thoughts are with Jo’s husband Brendan and their two young children. They will grow up without their mum, but can be immensely proud of what she did, what she achieved and what she stood for.
Other party MPs also expressed their sadness at the news.
Barry Sheerman, Huddersfield Labour MP, said:
I'm absolutely stunned, I was a mentor to her she was the same age as my daughter. She was one of the real talents in Parliament. People loved her in the constituency. This isn't a question of party politics, all of us have to talk about moderating our language.
Prime Minister David Cameron sent his condolences to Cox's family.
In a statement, he said:
This is absolutely tragic and dreadful news and my thoughts are with Jo’s husband Brendan, their two children and wider family. We’ve lost a great star. She had a big heart and people are going to be very, very sad at what has happened.
She was a very strong campaigning MP. She had a great track record of caring about refugees and had taken a big interest in how we can look after Syrian refugees and do the right thing in our world. She was a star for her constituents, a star in Parliament and a star right across the House.
It’s right that we are suspending campaigning activity in this referendum and everyone’s thoughts will be with Jo’s family and her constituents at this terrible time.”
The motive for the attack is not yet known. It has been reported that the man shouted "Britain First" before shooting her.
In an interview on LBC Radio Jayda Fransen, the deputy leader of Britain First, condemned the attack.
"This is just hearsay... it's not something we can condone at all. We think it's disgusting," she said.
Britain Stronger In Europe has suspended its campaign ahead of the European referendum vote next week.
We are suspending all campaigning for the day. Our thoughts are with Jo Cox and her family.— Open Britain (@Open_Britain) June 16, 2016
Vote Leave, the official campaign for an "Out" vote, also suspended campaigning.
In an official statement, a Labour spokesperson said: "We're aware of the incident that happened this afternoon. All our thoughts are with Jo Cox and her family. We cannot give any further updates at this time."
The full statement from West Yorkshire Police, issued earlier today, reads as follows:
At 12.53 today, police were called to a report of an incident on Market Street, Birstall, where a woman in her 40s had suffered serious injuries and is in a critical condition. A man in his late 40s to early 50s nearby also suffered slight injuries.
Armed officers attended and a 52-year-old man was arrested in the area. There are no further details at present. Police presence in the area has been increased as a reassurance to the community.
Condolences also came in from President of the European Commission Jean Claud-Juncker and President of the European Council Donald Tusk.
Deeply shocked by news of terrible attack against MP Jo Cox. Thoughts and prayers with her husband and children.— Jean-Claude Juncker (@JunckerEU) June 16, 2016
Repelled by tragic attack on British MP Jo Cox. My thoughts are with her family and loved ones— Donald Tusk (@eucopresident) June 16, 2016
Cox has been MP for Batley & Spen since 2015. She is married and has two young children, according to her website.
Her husband Brendan released a statement, which read:
Today is the beginning of a new chapter in our lives. More difficult, more painful, less joyful, less full of love. I and Jo's friends and family are going to work every moment of our lives to love and nurture our kids and to fight against the hate that killed Jo.
Jo believed in a better world and she fought for it everyday of her life with an energy, and a zest for life that would exhaust most people. She would have wanted two things above all else to happen now, one that our precious children are bathed in love and two, that we all unite to fight against the hatred that killed her.
Hate doesn't have a creed, race or religion, it is poisonous. Jo would have no regrets about her life, she lived every day of it to the full.
Before becoming an MP, Jo Cox worked for the charity Oxfam. In a statement it said:
Oxfam is deeply shocked to hear the news. Our thoughts and sympathies are with Jo and her family at this difficult time.