Undercover policing criticised in review
Shortcomings in how undercover police officers are deployed and monitored have been criticised by a review.
It says some senior officers have "poor" levels of knowledge of undercover policing and are failing to adapt to threats posed by online crime.
The report by Her Majesty's Inspectorate of Constabulary (HMIC) reveals that there are 1,229 undercover officers working in England and Wales.
The review was set up after controversy about ex-covert officer Mark Kennedy.
It discloses that 3,466 undercover operations were authorised between October 2009 and September 2013.
By Danny Shaw, BBC home affairs correspondent
The 206-page inspection report is a ground-breaking piece of work, partially lifting the lid on the "closed culture" of undercover policing.
The report reveals how many officers are deployed undercover and the number of operations - as well as exposing the many flaws in the system.
What emerges is that brave and courageous officers on the ground are being let down by patchy systems of psychological support and governance, while their bosses resist outside scrutiny and improvement: arguably the biggest gap is in the capability of forces to deploy covert officers to deal with online threats, such as child sexual exploitation and cyber-crime.
What inspectors hope is that this review will transform undercover policing, in the same way as the Macpherson Inquiry changed the police's approach to murder investigations.
'Slow to adapt'
The covert officers themselves are described in the report as "knowledgeable, professional and courageous".
Inspector Stephen Otter, who led the review, said: "It was disappointing to find inconsistencies and shortcomings in the way undercover officers were supported by policies, systems and training across the country.
"We were concerned by a generally poor level of knowledge and lack of expertise of those senior leaders who authorise the use of undercover officers."
Mr Otter also said that some police forces had been "slow to adapt" to online undercover tactics, and five forces were without any covert online capability.
Elsewhere, the report criticises the lack of psychological support in some forces, and calls for a combined 10-year cap on length of tenure for undercover officers, after it found that one officer had worked on undercover operations for more than 20 years.
Jack Dromey, the shadow minister for policing, said: "It is clear that systems governing such policing must be kept under strict control, which is why it is so worrying that HMIC has identified such a large variation between forces."
The report came about after the high profile case of former undercover officer Mark Kennedy, who spent a decade undercover, infiltrating groups involved in climate change protests.
He lived a double life as Mark Kennedy of the Metropolitan Police and as Mark Stone, green activist, based in Nottingham.
In January 2011, the trial of six green campaigners collapsed after he offered to give evidence on their behalf.
A further 20 activists had their convictions quashed on appeal following the officer's unmasking.
Mr Kennedy admitted in a newspaper interview to having two sexual relationships with female activists and said it was "a wrong thing to do".
He said in 2011: "The circumstances I was involved in led to that to happen and I can assure you that I am not the only person who has been involved in sexual relationships as an undercover officer."