Scottish police and prosecutors have confirmed the formal end of their four-year investigation in to phone-hacking.

Sources at the Crown Office and national force said their inquiry had come to an end withWednesday's acquittal of former News of the World editor Andy Coulson on perjury charges.

Mr Coulson was one of just four people reported to the Crown by police under their probe in to allegations of wrong-doing associated with the now-defunct Sunday tabloid.

He was the only one of the four to be taken to trial. Charges were dropped against fellow journalists Bob Bird, Douglas Wight and Gill Smith

Both police and prosecution sources have now confirmed there will be no further actions.

A police source said: "Op Rubicon is no longer an active Police Scotland inquiry. All the Rubicon cases we investigated were reported to the Crown."

A spokesman for the Crown said: "We received four reports from Police Scotland. These have all been dealt with."

Critics have claimed that the entire Operation cost £1m. Some 50 officers were involved in the Strathclyde operation, which began in 2011 and is understood to have started to wind down in 2013.

It has been two years since police made their last report, of Mr Wight, to the Crown.

Mr Coulson said his won prosecution was a "gross waste of public money". However, Operation Rubicon represents just a fraction of the costs of the entire investigation in to phone hacking across the UK.

By last year, the Metropolitan Police have spent £33m on parallel investigations in to phone-hacking, bribery of public officials and computer hacking, Operations Weeting, Elveden and Tuleta respectively.

These led to scores of arrests, of journalists and other figures associated with News International, and, in 2014, of the conviction of Mr Coulson for phone-hacking.

It was phone hacking that lay at the heart of Mr Coulson;s perjury case. The former editor - and No 10 spin doctor - had been accused of lying on oath about the practice during another perjury trial, that of ex-MSP Tommy Sheridan.

But a judge, Lord Burns at the High Court in Edinburgh, ruled Mr Coulson had no case to answer because his allegedly false testimony was not relevant to Sheridan's case.

The former politician was convicted of lying in a previous civil action to win damages from News of the World over stories about his sex life.

It was Sheridan's former lawyer, Aamer Anwar, who kicked off Rubicon. In July 2011 he handed over a dossier of information on phone-hacking to the police.

Mr Anwar said he had handed over "scores" of names of victims. Later it emerged that the phone of former First Minster Jack McConnell had been hacked. So too had that of Joan McAlpine, a SNP MSP who used to be deputy editor of The Herald.

Mr Anwar said: "It is of real concern that a spotlight on Andrew Coulson has meant Operation Rubicon failed to pursue other journalists in Scotland who engaged in criminality.

"Phone hacking and the illegal exchange of confidential information occurred on an industrial scale and certainly did not stop at the border. "Yet for some reason a decision was made not to pursue in Scotland the individuals named in this dossier despite over £1 million pounds being spent on an investigation."

Police Scotland had no comment on Mr Anwar's remarks.