Jo Cox, 41, was attacked by a man who then walked calmly away from the scene outside the library in Birstall, West Yorkshire, leaving his victim lying bleeding on the pavement. At least one witness heard the suspect shout "Britain first" before the shooting and during the arrest, Sky News and the Guardian reported. Police were understood to be investigating the claim.

Right wing, anti-immigrant and anti-Islam group Britain First denied any link to the shooting on their website, saying "Britain First obviously is NOT involved and would never encourage behaviour of this sort".

Both sides of the Brexit debate suspended campaigning for the day after hearing the news, and Prime Minister David Cameron cancelled a trip to Gibraltar.

In a statement, Labour leader Jeremy Corbyn said the whole of the Labour party, the Labour family "and indeed the whole country will be in shock at the horrific murder of Jo Cox". He praised her lifelong record of public service.

"Jo died doing her public duty at the heart of our democracy," he said.

"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."

Ms Cox's husband released a statement several hours after her death paying tribute to his wife's work and urging the public to fight against the "hatred that killed her."

"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 every day 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."

Suspect arrested by police after attack

Ms Cox had been holding a regular 'surgery' of one-on-one meetings with constituents at the library.

She was attacked by a man who "inflicted serious and sadly, ultimately fatal injuries", West Yorkshire police chief constable Dee Collins said.

The assailant then attacked another 77 year-old man, inflicting non life-threatening injuries.

An eyewitness told Press Association he heard screaming and saw "a guy who was being very brave and another guy with a white baseball cap who he was trying to control and the man in the baseball cap suddenly pulled a gun from his bag".

"He was fighting with (Cox) and wrestling with her and then the gun went off twice and then she fell between two cars and I came and saw her bleeding on the floor.

"The man stepped back with the gun and fired it and then he fired a second shot, as he was firing he was looking down at the ground."

Another witness told the BBC the attacker also lunged at Cox with a knife.

A 52-year-old man was arrested nearby shortly after the attack, West Yorkshire Police said, adding they were "not looking for anyone else in connection with this incident at this time". The suspect was elsewhere identified as Tommy Mair.

'This should not happen in Britain'

Former British Prime Minister Gordon Brown said he and his wife Sarah had worked with Jo and Brendan on charity campaigns for women and children.

"Whenever you talked to her, the compassion in her eyes and the commitment in her soul shone through."

"She she went to some of the most dangerous places in the world. The last place she should have been in danger was in her home town."

"People will say that this does not happen in Britain. This should not happen in Britain. And we must resolve that this will never again happen in Britain."

News of Ms Cox's death provoked a wave of sympathy from international leaders and politicians as well.

Former US congresswoman Gabrielle Giffords, who survived a shooting attack in 2011, was among those lending words of sympathy.

"Absolutely sickened to hear of the assassination of Jo Cox," she wrote on Twitter. "She was young, courageous, and hardworking. A rising star, mother, and wife."

Ms Cox had argued this week in a local newspaper that a "vote to leave is a risk that is simply not worth taking", and that a remain vote was "the patriotic choice".

The Labour MP's colleagues have described her as an eloquent and effective lawmaker. She was due to celebrate her 42nd birthday next Wednesday, and had been spoken of as a potential future minister.

According to the biography on her personal website, Jo Cox was born and raised around Dewsbury, England. She graduated from Cambridge in 1995, the first in her family to complete university.

She was involved in pro-European causes. She was an aide to Joan Walley, a British member of Parliament who supported European integration, and to Glenys Kinnock, a former Labour member of the European Parliament. Ms Cox has also worked with Oxfam, the anti-hunger charity; Sarah Brown, wife of the former Labour prime minister Gordon Brown, to reduce child and infant mortality; and the Bill and Melinda Gates Foundation, to combat modern slavery. She was married with two children, and lived part of the time on a boat on the Thames.

It was the UK's first murder of a serving MP in 26 years, since Conservative Ian Gow was killed by an IRA bomb outside his home

with The New York Times

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