Following is a summary of reaction:
Brendan Cox, Jo's husband
"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."
British Prime Minister David Cameron
"We have lost a great star. She was a great campaigning MP with huge compassion, with a big heart."
"It is 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."
Labour Party leader Jeremy Corbyn
"We've lost a wonderful woman, we've lost a wonderful member of Parliament, but our democracy will go on. Her work will go on. As we mourn her memory, we'll work in her memory to achieve that better world she spent her life trying to achieve."
Scottish First Minister Nicola Sturgeon
"This is utterly shocking and tragic news, which has left everyone stunned."
"She was held in huge regard as a brilliant young woman, who had already contributed a huge amount in her time in Parliament, and today she was simply going about her job as a local MP."
US Ambassador to Britain Matthew Barzun
"We are heartbroken by the loss to her family and country of MP Jo Cox. My love and our love to them, in this time of unbearable grief."
Former US congresswoman Gabrielle Giffords, who survived shooting in 2011
"Absolutely sickened to hear of the assassination of Jo Cox. She was young, courageous, and hardworking. A rising star, mother, and wife."
Max Lawson of Charity Oxfam, who worked closely with Cox
"Jo was a diminutive pocket rocket from the north. She was a ball of energy, always smiling, full of new ideas, of idealism, of passion. She gave so much to Oxfam."
David Miliband, former British foreign secretary
"People in need around the world have lost a tireless, effective and redoubtable champion today following the murder of Jo Cox MP. Her passionate advocacy, first of all working in NGOs and then in Parliament as an elected representative, on behalf of vulnerable and displaced people was a study in effective activism."
John Curtice, polling expert and politics professor at University of Strathclyde
"It's fairly clear no one is quite sure what has happened. Until it's clear who was responsible and what their motivation was or it might have been, all it does is stop the campaign when the 'Remain' side probably would not want it to be stopped."
Mujtaba Rahman, Europe Practice head at Eurasia Group
"This will hurt the momentum of the 'Leave' campaign, which has been gaining steadily in recent polls."
"It will allow British Prime Minister David Cameron an opportunity to act like a statesman and retrieve the agenda, something he has lost over the last week.
"If the incident is confirmed to have been motivated by Brexit, it will also reflect poorly on the more strident elements of the Vote Leave campaign, potentially swinging undecided voters towards 'Remain'."
Alan Ruskin, Global Co-Head of FX Research at Deutsche Bank
"Certainly people are talking about the possibility that this does influence the Brexit vote in favour of 'Remain'. It is a tragic event all around. There is a sense, there is an immediate emotional reaction, but there is still a week before the referendum itself."
"It definitely is seen as part of the story, the recovery of risk. Generally you are seeing so-called riskier assets recover. All the assets, whether equities, aussie/yen or sterling/yen are recovering. They are up on the perception of a higher probability of a 'Remain' vote."