Boris Johnson has insisted there is nothing “sleazy” about his messages with Sir James Dyson as he prepared to publish the communications in an attempt to cool a lobbying row.

The Prime Minister said the texts exchanged with the billionaire would be published later on Friday, as Downing Street declined to dismiss suggestions they had been leaked by former aide Dominic Cummings.

The leak revealed Mr Johnson had promised the entrepreneur he would “fix” a tax issue for Dyson staff working to develop ventilators early on in the coronavirus crisis.

The Prime Minister has faced questions over when he would publish his correspondence with Sir James after promising to do so under pressure in the Commons this week.

“Indeed, I think that’s happening today,” Mr Johnson told broadcasters during a visit to a farm in Stoney Middleton, Derbyshire, on Friday.

“But let me tell you, if you think that there’s anything remotely dodgy, or rum, or weird or sleazy about trying to secure more ventilators at a time of a national pandemic and doing everything in your power to do that then I think you’re out of your mind.”

Asked if he needs to rethink how he communicates with people, Mr Johnson said: “No. I think I need to maintain… to be in touch with people.”

Pressed whether he will keep his phone, after suggestions Cabinet Secretary Simon Case advised Mr Johnson to change his number over concerns about the ease with which lobbyists and business leaders were able to contact him, Mr Johnson smiled and said: “You need one these days.”

No 10 launched an internal inquiry led by the Cabinet Office into how messages between the Prime Minister and Sir James were leaked to the BBC.

But Downing Street sources have already begun pointing the finger at Mr Cummings, who quit as Mr Johnson’s senior adviser last year following a power struggle in No 10.

A No 10 spokesman declined to deny suggestions the controversial former aide is the suspect when repeatedly questioned during a Westminster briefing with journalists.

“I am not going to comment on speculation and, of course, it is for the Cabinet Office to take that work forward, I am not going to pre-empt it,” he said.

The Times, Daily Telegraph and Sun had all reported comments from an insider naming the controversial former aide as the suspected leaker.

“Dominic is engaged in systematic leaking,” a source told The Times.

“We are disappointed about that.

“We are concerned about messages from private WhatsApp groups which have very limited circulation.”

Labour leader Sir Keir Starmer criticised the “quarrelling” when asked about the reports during a campaign visit to Hartlepool.

“Now we’re getting arguments and quarrelling at the centre of Government about who is leaking information about privileged access,” he told reporters.

“Boris Johnson is desperate to try to put this at arm’s length, but it’s obvious that he can’t. This is government by WhatsApp, they’re all at it.”

The leak of the texts to the entrepreneur was not the first time the Prime Minister’s messages have been made public.

Mr Johnson was sent a text message by Saudi Crown Prince Mohammed bin Salman as a bid to buy Newcastle United FC ran into difficulties last June.

A No 10 source told The Sun that Mr Johnson “fears Dom was responsible for the text message leaks about James Dyson and Mohammed bin Salman”.

The former Vote Leave mastermind worked closely with Mr Johnson on the Brexit campaign and was a major figure in No 10 after the Prime Minister took office.

Mr Johnson stood by Mr Cummings when he found himself in the eye of a media storm after driving his family to County Durham during the coronavirus lockdown.

But Mr Cummings was subsequently ousted from Downing Street amid the fallout from an internal power struggle with the Prime Minister’s fiancee, Carrie Symonds.

No 10 had initially said there would not be a probe into how the exchange with Sir James was made public, but a change of course was announced on Thursday.

Mr Johnson earlier told the Commons during Prime Minister’s Questions that he was “happy to share all the details” of the exchanges as there is “nothing to conceal”.

The leaked text messages showed that in March last year Mr Johnson promised Sir James he would “fix” a tax issue for Dyson staff working to develop ventilators.

At the time, the businessman had been unable to get the desired assurances he was seeking from the Treasury over his Singapore-headquartered firm.

Sir James wrote to the Treasury requesting that overseas staff would not have to pay additional tax if they came to the UK to work on the ventilator project.

But when he failed to receive a reply, Sir James reportedly took up the matter directly with the Prime Minister.

He said in a text that the firm was ready but that “sadly” it seemed no-one wanted them to proceed, to which Mr Johnson replied: “I will fix it tomo! We need you. It looks fantastic.”

The Prime Minister then texted him again saying: “(Chancellor) Rishi (Sunak) says it is fixed!! We need you here.”

Two weeks later, Mr Sunak told the Commons Treasury Committee that the tax status of people who came to the UK to provide specific help during the pandemic would not be affected.