Boris Johnson has been warned that he could lose public funding for legal advice if he tries to “frustrate or undermine” the government’s position on the Covid-19 inquiry.

Cabinet Office lawyers told the former prime minister that money would “cease to be available” if he breaks conditions such as releasing evidence without permission, the Sunday Times reported.

Johnson has been at the centre of a row as ministers launched a high court bid to challenge the inquiry’s demand for his unredacted WhatsApp messages and contemporaneous notebooks.

He said he would send all his messages to the official investigation directly, circumventing the Cabinet Office.

Last week, the Times reported that Johnson sent 300 pages of unredacted WhatsApp correspondence to the inquiry after the government began a judicial review designed to block the disclosure of his messages without prior vetting by officials and to ensure Whitehall has the final say on what is handed over.

The Sunday Times detailed a letter sent by Cabinet Office lawyers to Johnson last week.

“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 said.

The letter 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”.

The Cabinet Office said the letter was “intended to protect public funds” so that taxpayer-funded lawyers are not used for any purpose other than aiding the inquiry.

Tory donor Lord Cruddas, an outspoken backer of