The former prime minister was told in a letter last week that money would “cease to be available” if he broke any of their conditions.
The Cabinet Office has said the letter makes clear “Mr Johnson has a duty to provide sincere witness,” to the inquiry, which could include him sending witness statements and requested documents.
The letter, reported in the Sunday Times, was said not to be in response to “any recent event”. It, however, follows ministers launching a High Court bid to challenge the inquiry’s demand for his unredacted WhatsApp messages and notebooks.
Mr Johnson announced in May 2021 there would be an inquiry into the government’s preparation and management of the Covid-19 crisis. The result of the investigation has seen legal teams look through thousands of documents. It could be revealing with a lot of questions unanswered and the reputations of those involved at stake.
The Cabinet Office’s pressure to cooperate could continue an already rocky road forward for Mr Johnson with the Sunday Times suggesting he may already have fallen foul of its requirements.
Mr Johnson had vowed to send all his messages to the official investigation directly, bypassing the Cabinet Office and Rishi Sunak.
A Cabinet Office statement sent to The Independent read: “This letter from officials simply reiterates that taxpayer-funded lawyers must be used to aid the Covid Inquiry and for no other purpose.
“The letter makes clear Mr Johnson has a duty to provide sincere witness to the inquiry independently and without reference to the views of the current government.
“This letter was intended to protect public funds. It in no way prevents Mr Johnson from providing whatever evidence he wants to.”
The office added that the letter, which was sent last week, was not seen or signed off by ministers.
The letter reportedly stated: “The funding offer will cease to be available to you if you knowingly seek to frustrate or undermine, either through your own actions or the actions of others, the government’s position in relation to the inquiry unless there is a clear and irreconcilable conflict of interest on a particular point at issue.”
It added that funding would “only remain available” if he complied with conditions such as sending the Cabinet Office “any witness statement or exhibit which you intend to provide to the inquiry so that it can be security checked by appropriate officials”.
Former culture secretary Nadine Dorries, a staunch ally of Mr Johnson, said it was “not a good look for the government”.
“All evidence provided should be unfettered and not restricted by gov censorship – whatever form that may take,” she tweeted.
Tory donor Lord Cruddas, an outspoken backer of Mr Johnson, urged the MP not to be “held to ransom” by the threat.
“Don’t worry @BorisJohnson I can easily get your legal fees funded by supporters and crowd funding, it’s easy,” he tweeted.
After the government launched its legal battle, Mr Johnson wrote to the inquiry’s chairperson, Baroness Hallett, saying he was sending all the unredacted WhatsApps he had given to the Cabinet Office.
He said he would like to do the same for the messages on an old phone he was told not to use after it emerged the number had been available online for 15 years.
That device will be crucial, containing discussions before May 2021 including around the three national lockdowns he ordered.
Mr Johnson told the chairperson that he was “not willing to let my material become a test case for others when I am perfectly content for the inquiry to see it”.
The Cabinet Office missed Lady Hallett’s deadline set on Thursday to hand over the requested material.
But the government department has been trying to resist the publication of messages it believes are “unambiguously irrelevant”.