The arrest of Tory frontbencher Damian Green as part of an inquiry into Home Office leaks was "not proportionate", an official report has found.
How did this all start?
Mr Green, the shadow immigration minister, was arrested and held by the Metropolitan Police for nine hours on 27 November last year. His offices in Kent and in the Houses of Parliament, and his home were also searched. He was later released on bail.
Who is Damian Green?
Firmly on the moderate wing of the Conservative Party, the Ashford MP is a former television journalist and adviser to John Major who entered Parliament in 1997. He has held the immigration brief since 2005.
Why did the Metropolitan Police arrest him?
As part of an investigation into alleged leaks from the Home Office, a junior Home Office official was suspended over a series of items of sensitive information and the matter was referred to the police.
Christopher Galley, who passed on information to Mr Green, was arrested on 19 November but released on bail without charge. The police said they had arrested Mr Green on suspicion of "conspiring to commit misconduct in a public office" and "aiding and abetting, counselling or procuring misconduct in a public office".
When they questioned him, they are said to have suggested to him that he had not "simply received leaked" information but had "groomed" a civil servant to pass it to him. Neither Mr Galley nor Mr Green - who have both denied any wrongdoing - will now face prosecution, the CPS has announced.
Who authorised the arrest?
The then Acting Met commissioner Sir Paul Stephenson - who is now the permanent head of the force - said the decision had been taken by a team of officers under the command of assistant commissioner Bob Quick. Mr Quick resigned earlier this month over a security blunder - he was photographed with a secret terrorism-related document on show when he arrived for a Downing Street briefing.
What about the Commons search?
Speaker Michael Martin said one of his staff, the Serjeant-at-Arms Jill Pay, had allowed the police into Mr Green's office without a warrant. Instead, she signed a consent form allowing the action. Mr Martin was told a search of Mr Green's office might take place several hours before it happened - but he insisted he had not been told the police did not have a warrant.
How serious were the leaks?
The Commons Home Affairs Select Committee has published a report saying civil servants "exaggerated" the seriousness of Home Office leaks which led to Mr Green's arrest, possibly out of "frustration" at the situation. It had been "unhelpful to give the police the impression that the Home Office leaker had already caused considerable damage to national security", it added.
Why are MPs angry about a police raid on Parliament?
Some believe the police breached parliamentary rules put in place after Charles I sent soldiers to arrest MPs in 1642. On that occasion the Speaker refused to co-operate. Other MPs have raised concerns that police might be able to seize material given to them by their constituents, including information critical of the police. The Tories said the material police were searching for had been used in the Commons by Mr Green, and therefore should be covered by parliamentary privilege and not be used in a prosecution.
What was leaked to Mr Green?
There have been a number of high-profile leaks of information from the Home Office, stretching back to 2007, including:
• An e-mail from Home Secretary Jacqui Smith's private secretary showing she had chosen not to publicise the fact that licences had been granted to security guards who were illegal immigrants. She denied there had been a "blunder" but later admitted as many as 11,000 illegal immigrants may have been cleared to work as security staff.
• A memo to Home Office minister Liam Byrne - now the Cabinet Office minister - revealing an illegal immigrant had been employed as a cleaner in the Commons.
• A draft letter from the Home Office to Downing Street in which Ms Smith warned a recession could lead to a rise in crime. The document contained draft advice not cleared by Ms Smith.
Has what happened to Mr Green happened before?
No. It is unprecedented for an MP to be arrested and his office searched by police in connection with a leak inquiry. Mr Green was suspected of committing a common law offence under an obscure and little-used piece of legislation.
Why are there fears about its impact on democracy?
There is concern the police are being used to silence critics of the government and prevent disclosure of embarrassing material - something firmly denied by ministers. Opposition parties and journalists have long relied on leaks of sensitive material to help them do their job of holding the government to account. Many internal leak inquiries have been launched over the years, but the perpetrators are rarely caught. The police have occasionally become involved but successful prosecutions have been very rare.
What reason did the CPS give for dropping the case?
Director of Public Prosecutions Keir Starmer said there was a "high threshold before criminal proceedings can properly be brought", and that he had considered the "freedom of the press to publish information and ideas on matters of public interest". He said the information leaked was not secret information or information affecting national security and there was "insufficient evidence for a realistic prospect of conviction against Mr Galley or Mr Green".
What was Mr Green's reaction?
Speaking outside Parliament, the MP said: "One of my jobs as Conservative immigration spokesman is to expose the many failings of the government's immigration policy. That's precisely what I was doing in this case and that's why ministers were so embarrassed."
What does the government say?
Then Home Secretary Jacqui Smith said it was her job to protect the British people and the sensitive information about them, which is what she has done. Earlier, she said no government minister had known Mr Green was the subject of a police investigation or that the arrest was going to happen. She added that she "wholeheartedly" supported the rights of MPs to hold governments to account. However, the "systematic" nature of leaks had threatened the integrity of the Civil Service, she added.
The Speaker appointed a committee of seven senior MPs to investigate the issue and ruled that, in future, no MP's office could be searched without a warrant.
What about the police?
Ex-British Transport Police chief Ian Johnston was asked by the Met to review the investigation. His report concluded that Mr Green's arrest was "not proportionate", that the leaks resulted only in "embarrassment" for ministers and that they were not "likely to undermine government's effectiveness".