Just over two months ago the Guardian published the story of Milly Dowler's phone – and how it was hacked by a private investigator working for the News of the World after the teenager's abduction and murder. It was a revelation which caused worldwide revulsion and outrage. It led to resignations, parliamentary debates, official inquiries and humble corporate apologies. A newspaper was closed and News Corp's bid to take control of BSkyB was stopped in its tracks by a unanimous vote of parliament. The former Metropolitan police chief Sir Paul Stephenson was gracious enough to praise the Guardian's role in persisting where three police inquiries had failed. The country should be grateful, he said, that this paper ignored his own attempts to warn us off. Only this week the culture secretary, Jeremy Hunt, described the Guardian's campaign as "investigative journalism of the highest quality".

Incredibly, the Metropolitan police are now trying to find out the source of the Milly Dowler story. To that end they are – quite extraordinarily – using the Official Secrets Act to try and force the Guardian to hand over documents which would betray our sources. Papers served on the newspaper this week demand that, within seven days, our reporters – including Amelia Hill and Nick Davies, who relentlessly covered the phone-hacking story for more than two years – hand over anything that could lead the police to identify who blew the whistle on the Dowler story and others.

It beggars belief that the Metropolitan police – who, for years, declined to lift a finger against News International journalists despite voluminous evidence of criminal behaviour – should now be using the Official Secrets Act to pursue the Guardian, which uncovered the story. The Official Secrets Act is a very powerful sledgehammer and the police have no business using it to try to defeat the defences that journalists would normally rely on to prevent them – or anyone else – from trying to expose confidential sources. The only known previous attempt to use the 1989 act against a journalist – the writer Tony Geraghty – collapsed, as did a similar police threat (not using the OSA) to prosecute the Conservative MP Damian Green for "aiding and abetting" misconduct in a public office.

Operation Weeting seemed finally to be doing something to restore the Met's reputation, which has been so tarnished by the handling of this whole affair. But these heavy-handed tactics will alarm every editor and reporter in the country. If the police succeeded with the Official Secrets Act in this case it would be a very bleak day for one of the fundamental freedoms that underpins democracy itself.