Brokers at NYSE
Stock brokers at the New York Stock ExchangeGetty

Trading on the New York Stock Exchange resumed after an almost four-hour shutdown on 8 July after it failed to cope with what operators described as a technical glitch.

The shutdown came only hours after United Airlines temporarily grounded all its flights following what it described as a technical issue.

The White House and the Department for Homeland Security swiftly denied reports that the NYSE or United had come under a cyberattack.

White House press secretary Josh Earnest told reporters that President Barack Obama had been informed of the situation, but there was "no indication that malicious actors were involved".

Mary Jo White, the chair of the Securities Exchange Commission, said the regulator was also "closely monitoring the situation".

U.S. Homeland Security Secretary Jeh Johnson said the glitches by United Airlines and the NYSE were apparently not related to "nefarious" activity.

"I have spoken to the CEO of United, Jeff Smisek, myself. It appears from what we know at this stage that the malfunctions at United and the stock exchange were not the result of any nefarious actor," Johnson said during a speech at Washington think tank the Center for Strategic and International Studies.

"We know less about Wall Street at this point, except that their system is in fact up again," he added.

When the shutdown began, Peter Costa, a trader on the floor with Empire Executions, told the New York Times that exchange employees manually cancelled about 700,000 orders that were in the system.

"I was about to order my lunch and when I turned around the screens all went blank," Costa said.