LONDON -- The British government has refused an order to hand over a sheaf of former Prime Minister Boris Johnson's personal messages to the country's COVID-19 pandemic inquiry. It said Thursday it would try to challenge the order in court, setting up an extraordinary legal battle with an inquiry that Johnson himself set up.

The notebooks, diaries and WhatsApp messages between Johnson and other officials form key evidence that the head of the probe, retired judge Heather Hallett, wants to see. The government has handed over incomplete versions, saying it cut personal and private information that is "unambiguously irrelevant" to the investigation.

Hallett -- who has the power to summon evidence and question witnesses under oath -- wants to judge for herself, and set a deadline of 4 p.m. (1500 GMT) Thursday for the government to hand over the unredacted documents, covering a two-year period from early 2020.

Soon after the deadline passed, the government said it would seek to challenge the order in court.

"The Cabinet Office has today sought leave to bring a judicial review" of the decision, it said. "We do so with regret and with an assurance that we will continue to cooperate fully with the inquiry."

Just before the deadline expired, Prime Minister Rishi Sunak said his government would "comply, of course, with the law and cooperate with the inquiry."

"We are confident in our position but are carefully considering next steps," he said.

WhatsApp is a favored way for British politicians, officials and journalists to converse. The tone is often candid or casual, and potentially embarrassing. The government is worried about the precedent that disclosing Johnson's full, unredacted conversations might set.

Hallett, however, said "the entire contents of the specified documents are of potential relevance to the lines of investigation being pursued by the inquiry."

The issue has caused tension between Johnson and Sunak's administration, which claimed this week that it did not have the material Hallett wanted. Both are Conservatives, but Sunak has tried to distance himself from the chaos that engulfed the government during Johnson's scandal-plagued three-year term in office.

On Wednesday, Johnson's office said the former prime minister had given the government all the material and urged authorities to hand it to the inquiry.

The U.K. has recorded more than 200,000 deaths among people testing positive for COVID-19, one of the highest tolls in Europe, and the decisions of Johnson's government have been endlessly debated. Johnson agreed in late 2021 to hold an inquiry after pressure from bereaved families.

Hallett's inquiry is due to investigate the U.K.'s preparedness for a pandemic, how the government responded and whether the "level of loss was inevitable or whether things could have been done better."

Public hearings are scheduled to start June 13, and to last until 2026. U.K. public inquiries are often thorough, but rarely quick. An inquiry into the 2003 Iraq war and its aftermath began in 2009 and issued its 2.6-million word report in 2016.

Johnson is among the senior officials due to give evidence.

The inquiry has already landed Johnson in hot water. He was one of dozens of people fined last year for breaking his own government's pandemic lockdown rules in the so-called partygate scandal. Earlier this month, government-appointed lawyers helping Johnson prepare his submissions and testimony came across evidence of more potential breaches of COVID-19 restrictions. Civil servants reported the information to police, who say they are assessing the new evidence. Johnson denies wrongdoing.