An investigation into alleged computer hacking by a private investigator will not pursue further action against anyone, Metropolitan Police says.

Operation Kalmyk looked at computer hacking offences in relation to suspected business espionage.

As part of Kalmyk, which began in 2011, 22 people were interviewed under caution, 15 of them under arrest.

The Crown Prosecution Service has now told Met Police there was insufficient evidence for any convictions.

The Met had been consulting with the CPS throughout the course of the Operation Kalmyk investigation, which looked into the investigator's alleged offences that were said to have been carried out on behalf of other clients.

The CPS has said it considered charging 15 suspects under the Computer Misuse Act.

But on Wednesday, it said that had not been possible because the alleged offences were said to have been committed between 2005 and 2007, when the law included a six-month time limit for starting a prosecution.

The BBC's home affairs correspondent Tom Symonds says the high-profile inquiry was triggered in 2011 by a BBC Panorama investigation.

This alleged a private investigator had obtained access to emails belonging to a soldier, Ian Hurst, who worked in army intelligence in Northern Ireland.

The programme claimed two private investigators targeted Mr Hurst because of secrets he was keeping dating back to his time working with an undercover unit that handled IRA informers.

Panorama said the hacking was ordered by Alex Marunchak, an executive at the News of the World.

Mr Marunchak, who was arrested and questioned as part of the computer hacking inquiry said he was "relieved for my family who have been through an awful lot while this witch-hunt has been going on".

He added: "It's a monumental waste of money. I imagine Scotland Yard has got better things to spend money on than that."

Alleged hacking victim Mr Hurst has immediately asked the CPS to review its decision not to prosecute.

He claimed police became aware he had been targeted in 2006, rather than 2011, as a result of a surveillance operation examining possible police corruption.

Mr Hurst also said action could have been taken much earlier, avoiding any time limit and resulting in a prosecution.

The Metropolitan Police said in response: "In 2006, intelligence was received during a sensitive investigation.

"The intelligence was recorded but not acted upon. We have not been able to find any documented decision-making as to why no action was taken."

The force said no further details could be given "due to the circumstances of the intelligence being received".

Operation Kalmyk was investigated as part of Operation Tuleta, which began in the wake of the phone-hacking scandal and is being run by the Metropolitan Police.

Police said the CPS decided there was sufficient evidence to charge one person with an offence contrary to the Fraud Act but that it would not be in the public interest to do so.

The Met said in a statement that it understood that the complainants may be "very disappointed" by the decision.

"However, we respect and fully appreciate the CPS's decision and recognise that in this case there were complex legal and evidential issues that meant there could not be a realistic prospect of conviction," it added.