Florida governor Ron DeSantis has extended the voter registration deadline after unexplained heavy traffic crashed the state’s online system and potentially prevented thousands of people from casting ballots in next month’s presidential election.

Mr DeSantis will extend the deadline that expired on Monday until 7pm local time on Tuesday.

His announcement came shortly after a state official told the Associated Press that at times more than half a million attempts an hour had hit the system on Monday.

Mr DeSantis also ordered election, motor vehicle and tax collection offices to stay open until 7pm for anyone who wants to register in person.

“You can have the best site in the world, but sometimes there are hiccups,” he said during a press conference at a retirement community in central Florida. “If 500,000 people descend at the same time, it creates a bottleneck.”

Florida secretary of state Laurel Lee, who oversees the voting system, said in a statement that authorities “will work with our state and federal law enforcement partners to ensure this was not a deliberate act against the voting process”.

The FBI and the Cybersecurity and Infrastructure Security Agency warned elections officials nationwide last week that cyberattacks could disrupt their systems during the run-up to the election.

They particularly noted distributed denial-of-service (DDoS) attacks, which inundate a computer system with requests, potentially clogging up servers until the system becomes inaccessible to legitimate users.

The potential for outside meddling is an especially sensitive issue in Florida, a key battleground state in November’s election between President Donald Trump and his Democratic challenger, former vice president Joe Biden.

The state has lingering questions about Russian hacking during the election four years ago. Last year, state officials confirmed that election-related servers of at least two Florida counties were breached by Russian meddlers. No votes or records were tampered with.

This is not the first major computer shutdown to affect the state government this year. For weeks in the spring, tens of thousands of Floridians who lost their jobs because of the coronavirus pandemic could not file for unemployment benefits as repeated crashes hit an overwhelmed computer system.

Mr DeSantis replaced the director overseeing that system but blamed the problems on his predecessor, fellow Republican Rick Scott, who is now a US senator.

Democrats jumped on the latest issue, saying it and the unemployment fiasco show that the DeSantis administration is inept and accused it of trying to stop people from voting.

A civil rights group had threatened to sue if the governor did not extend the deadline.

The Lawyers’ Committee for Civil Rights Under Law said the breakdown would unjustly deprive thousands of ballots for president and other offices.

Kristen Clarke, the group’s president, said it sued Virginia in 2016 after its computer system crashed just before the deadline, winning an extension that allowed thousands of additional voters to register.