Undercover police officers can never be authorised to start sexual relationships with those they are targeting under official guidelines published for the first time.
The instructions also state that taking drugs cannot be signed off as a tactic for covert units.
In an unprecedented move national guidance on undercover policing is being made public by the College of Policing.
It states that it is "never acceptable" for an undercover operative to "form an intimate sexual relationship with those they are employed to infiltrate and target or may encounter during their deployment", adding: "This conduct will never be authorised, nor must it ever be used as a tactic of a deployment."
Shadowy police teams have been at the centre of controversy in recent years following a flurry of claims, and Scotland Yard has made payouts to several women who unwittingly became involved in relationships with undercover officers.
A judge-led inquiry into undercover policing in England and Wales was launched last year.
The guidance says that if an undercover officer engages in unauthorised sexual activity for whatever reason - for example they perceive an immediate threat to themselves or others if they do not do so - then this "will be restricted to the minimum conduct necessary to mitigate the threat".
This would then be recorded, reported and the circumstances investigated.
The position set out on sexual activity was described as a "much stronger and unequivocal statement" compared to previous guidelines.
Alex Marshall, the organisation's chief executive, said: "It can't be authorised. It's wrong, it shouldn't happen.
"If in some extreme circumstance something happens where the operative has gone outside this guidance then you have to report it and it will be investigated."
Conduct may be authorised that involves "communications of a sexual nature" where the authorising officer believes it is "necessary and proportionate to operational objectives", the guidance adds.
Taking controlled drugs "will not be authorised as a tactic of a deployment" but if an officer does so because they perceive an immediate threat this should be limited to the minimum extent necessary to mitigate the threat.
The 80-page document also says that for an undercover deployment to be effective it may be necessary for personnel to participate in criminal activity about which they have been tasked to report.
It warns all undercover unit staff should be aware of the "dangers" posed through "exposure in true identity" on social media networks.
Undercover policing is used by forces across England and Wales to obtain evidence and intelligence. "Foundation" operatives carry out low-level infiltration, for example buying drugs on the street.
An "advanced" operative is trained to undertake deployments involving higher-level infiltrations in which they must be able to withstand intense scrutiny, such as counter-terrorism work.
The draft guidance is being published today at the start of a consultation, with the final version released later this year.
It says operatives can only work once they have been "accredited" and should undergo psychological or personality assessments.
Mr Marshall described undercover policing as an "essential tactic" used to protect the public, save lives and bring serious and organised criminals to justice.
He said: "By publishing the vast majority of the guidance, withholding only operational tactics which would no longer be viable if shared, we want the public to see the measures we have in place to ensure undercover policing is used in a way that is proportionate, lawful and ethical."
Lawyer Jules Carey, of Bindmans, who is representing individuals affected by undercover policing, welcomed proposed controls on recruitment, authorisation and oversight.
"It is particularly good to see that the guidance states that intimate sexual relations will never be authorised, nor ever used as a tactic again," he said.
However, he continued: "It is disappointing that the guidance fails to spell out that in a democracy the first consideration should be whether it is necessary to use an undercover officer at all, or whether the intelligence could be obtained through some other means.
"The guidance should also make it clear that the degree of intrusion should be proportionate to the seriousness of the crime being investigated."