The Prime Minister said he does not want to see journalists prosecuted for doing what is in the public interest amid alarm that a review of the Official Secrets Act could curtail investigative reporting.

Fears have been raised that a Home Office consultation into updating the 1989 Act could lead to reporters who are given leaked documents being treated similarly to spies.

Investigative journalists could face jail sentences of up to 14 years under planned changes that the industry has said could “criminalise” the press for upsetting the current and future governments.

The Sun newspaper said exposes such as the last month’s damaging revelations about Matt Hancock’s relations with his aide, which broke coronavirus social distancing rules and led to his resignation as health secretary, could become things of the past if the changes are brought in.

Whatever this thing is, I don’t for one minute think it is going to interrupt the normal process

But in an interview with LBC radio, Boris Johnson said he did not think “for one minute” that the alterations, designed to account for shifting threats in the digital age, could prevent journalists from carrying out investigations.

The former journalist said: “We don’t – I don’t – want to have a world in which people are prosecuted for doing what they think is their public duty and… in the public interest.

“I’m full of admiration for the way journalists generally conduct themselves.

“Whatever this thing is, I don’t for one minute think it is going to interrupt the normal process.”

Mr Johnson said many of the great exposes in journalism, such as Watergate in the US or the thalidomide drug scandal over treatment given to pregnant women, stemmed from “tainted sources”.

In 1988, the Conservative Party leader was himself sacked by The Times after he made-up a front-page quote.

Speaking to LBC, the former Spectator editor added that “one man’s treacherous betrayer of confidences and irresponsible leaker is another man’s whistleblower” as he defended journalists.

He said most behaved with “great responsibility” when it came to handling information that should “not be put into the public domain”.

But, asked whether the consultation on the Official Secrets Act should be “ripped up”, Mr Johnson suggested the review of the feedback should continue given it is already under way.

The issue was raised with No 10 last week by reporters during one of the daily media briefings.

A spokesman for the Prime Minister said: “You’ve heard the Prime Minister speak before about the vital role the press plays in being allowed to investigate things that are in the public interest.

“We are clear that freedom of the press is an integral part of the UK’s democratic process, which is why we’ll always be committed to ensuring the right balance is struck between protecting press freedoms and the ability of whistleblowers to hold organisations to account.

“But as you know, this is a consultation and, as we do with all consultations, we’ll consider all the responses carefully before we make any decisions.”

Asked whether that left the door open to a public interest defence being included in the updated Act, the spokesman added: “It is a consultation so it is important we allow that to run its course and we study the responses closely before we set out any more details in due course.”

A Home Office spokesman said: “The consultation was started with a purpose, so we will be listening closely to what feedback there has been.

“We will announce the results in due course.”