The prime minister immediately abandoned plans for a rally in Gibraltar, George Osborne also changed his Mansion House speech at the last minute to reflect on Cox's life and work. Labour Leave and Economists for Brexit also scrapped their latest engagements today as a mark of respect.
It also looks increasingly likely that parliament will be recalled so that fellow MPs can pay tribute to Cox, a human rights campaigner who was elected to the House of Commons at last year's general election. The House is currently in recess until after the EU referendum and is scheduled to return on 27 June. David Cameron was said to be considering the calls and a final decision is expected this afternoon.
With the suspension of campaigning and MPs set to return to Westminster, financial markets moved higher as they expected the heat to be taken out of the debate in the final few days - something which could help the Remain side.
With the tragic events occurring just seven days before the vote, there was even speculation that the referendum could be pushed back.
Could the referendum be delayed?
Experts in constitutional law and parliamentary procedure told City A.M. there was no legal reason this should not be an option.
"The referendum is legally set out in the EU Referendum Act of 2015," said Paul James Cardwell, a reader in EU Law at the University is Sheffield.
"[But] it only says that the date must be no later than the end of 2017".
"Legally, anything is possible, because ultimately it's up to parliament. Just as we saw with the extension of registration, parliament is sovereign. Legally, I don't think there's a problem with [changing the date] if the procedures are followed."
However, he added: "The consequences are more political."
How would the referendum be delayed?
Because the specific date of the vote - 23 June - is set out in something called "regulations", approved by Parliament through "delegated legislation" as opposed to the original Act of Parliament, the barriers required to jump over in order to change the date are slightly lower.
Acts of Parliament mandate the customary ping-pong between the Houses of Commons and Lords, whereas secondary legislation allows changes to be passed at short notice without such time for scrutiny. The extension of the voter registration deadline last week, for instance, was pushed through in less than two days.
Perhaps the biggest complication from a technical point of view, if a delay is on the table, would be resolving the fact many postal votes will already have been returned.
However, since such action would still require "legislative action", unless it won the support of both campaign groups and - probably - both Conservative and Labour MPs, it is unlikely to materialise.
Will the referendum be delayed?
Professor Robert Hazell from the Constitution Unit at University College London (UCL), said it was "very unlikely [the] referendum will be delayed".
There is "no precedent for delaying a referendum," he added, suggesting the closest thing would be the "delay of local elections in 2001 by a month because of foot and mouth disease".
Any extra changes to the regulations which set out the date for the referendum would also be subject to judicial appeal, the University of Sheffield's Cardwell pointed out. Brexit campaigner Arron Banks has already said he is considering legal action over the extension of the voter registration deadline - something which threatens to bring the result of the entire referendum into question.
With the campaigns still on hold, Britain Stronger in Europe and Vote Leave are currently deciding what to do next. The ultimate decision is up to the government, however, and official sources said there were no plans to delay the vote.
The short answer, then, is no: the referendum almost certainly won't be delayed. But - technically, legally and hypothetically - it is a possibility.