London police were accused on Monday of breaching the human rights of three journalists working for Britain's top-selling newspaper by accessing their phone records in a bid to find the source for a story which brought down a senior minister.
Detectives from London's Metropolitan Police used the covert power to discover who had leaked a police report to journalists from the Sun tabloid in 2012 which revealed former minister Andrew Mitchell had sworn at and insulted officers on duty outside Prime Minister David Cameron's Downing Street office.
The three reporters have taken their case to the Investigatory Powers Tribunal (IPT), a body which examines complaints from the public about unlawful use of covert techniques by public authorities
The landmark case also comes at a time Britain is wrestling with a high-profile debate over the surveillance powers of the police and spies, and whether they infringe on the public's privacy.
The Sun had reported that Mitchell, who title was Chief Whip, responsible for keeping discipline among legislators in Cameron's Conservative Party, had referred to police as "plebs" after they refused to allow him to cycle his bike through the main gates of Downing Street.
He denied using that insulting term but did later apologise for swearing.
In the ensuing row, Mitchell was forced to resign and last year lost a libel case against the paper and the police officer involved in the confrontation.
Four other officers, who had not been present at the time but were responsible for the leak or fabricating evidence about the incident, were sacked with one being jailed.
Detective Chief Inspector Tim Neligan, who led the police inquiry, told the IPT officers had been under "considerable pressure" from senior political figures and others to find out whether there had been a conspiracy to get Mitchell sacked.
He defended the decision to access the reporters' phone records saying it was the most appropriate way of finding the "missing piece of the jigsaw" which was the source of the leaked story.
However, Gavin Millar, the journalists' lawyer who said the case was the first of its kind, said the actions violated journalists' fundamental rights to protect confidential sources.
He questioned why Neligan had not used other methods, which would have required judicial oversight, to obtain the information.
The IPT is due to last two days with the verdict likely later in the year.