Scotland Yard has refused to disclose the number of times it has seized journalist phone and email records without their consent.

Pressure on the Metropolitan police is mounting to reveal why they chose not to invoke the law which guarantees confidentiality of journalistic sources when investigating a police mole in the Plebgate saga.

It emerged earlier this week that the police had ordered Vodafone to hand over the phone records of the Sun’s political editor using the Regulation of Investigatory Powers Act (Ripa), which does not offer journalistic privilege protecting names of sources.

The first Tom Newton Dunn and his bosses at the Sun knew of the disclosure was when the Met’s report into Plebgate, which included a paragraph disclosing the fact they had analysed his phone records, was published on Monday.

Normally police are required to go to court using a different law under the Police and Criminal Evidence Act (Pace) to request journalistic material.

Unlike Ripa, under Pace the police are required to notify journalists or their media organisation of the court application.

Asked how many times they have used Ripa to seize telephone or email records belonging to journalists, Scotland Yard said: “We are not prepared to discuss.”

Asked why they did not use Pace to seize Newton Dunn’s records, a spokesman said: “We use the most appropriate legislation. Ripa was the most appropriate and lawful means of obtaining this data that was essential to progressing a criminal investigation into allegations of corruption, specifically that police officers were conspiring to bring down a cabinet minister.”

The Met added that “in order to obtain such an authority under Ripa officers must demonstrate that it is proportionate, legal and necessary” and that in this case the authorising officer, who must be of superintendent rank or above, was “independent from the investigation team”.

Newton Dunn had resisted requests to reveal the sources behind his Plebgate story of September 2012 and had been threatened with arrest.

The Sun has said it was “concerned” to learn that the police had then gone on to seize his records through this secret means.

The Sun said it was writing to Sir Anthony May, the interception of communications commissioner, to examine how many times the police had sought authorisation for journalistic records this way.