West Yorkshire Police issued a letter to Predator HunTers saying that it is unable to sanction the group’s activities regarding the identification of potential child sex abusers.
It asks them to cease posing as children online, engaging with potential offenders and arranging to meet them.
Concerns highlighted include that the group’s actions could interfere with ongoing police investigations.
But the group seemed undeterred when it tweeted a picture of the letter in response, saying the force was not fit for purpose.
It added: “We catch paedophiles because you don’t have the man power or funds, yet you try stopping us.”
West Yorkshire Police said the letter had been issued in line with National Police Chiefs’ Council policy but said it could not comment further on the specific reasons.
A spokeswoman said: “We would always encourage members of the public to report any illegal activity relating to child sexual exploitation which they come across during normal online activity.
“While the force does, of course, share the concerns of these groups regarding the internet activity of potential child abusers and the danger they present, it must be stressed that the activities they seek to engage in are the responsibility of law enforcement agencies. Due to this we ask them to refrain from this ‘hunter/exposure’ style activity.”
The biography on the Predator HunTers Twitter account, which was set up in April, reads: “We are sexual predator hunters that protect our children online.”
In the picture of the letter, the name of the recipient has been blocked out and the location of the account holder is listed only as England.
But the police spokeswoman cited one of the force’s concerns as being that the group’s activities could pose a risk to children in West Yorkshire by inviting potential offenders into the area.
She also reiterated points made in the letter that the activities of the group could interfere with police investigations and the evidence gathered by the group could be excluded from prosecutions if it was ruled to have been illegally obtained.
The spokeswoman said: “Law enforcement officers who undertake undercover or covert activities are specially trained to do so and work under strict authorities provided within legislation.
“This process ensures, not only that the evidence is lawfully collated but also that the human rights of all parties are met.
“West Yorkshire Police fully investigates all allegations of child sexual exploitation and has dedicated, specialist safeguarding officers in all our policing districts.
“We urge members of such ‘exposure’ groups not to carry out this activity and instead to report suspected offences in West Yorkshire to the force."
But a spokeswoman for Predator HunTers said more senior legal authorities had declared that the activities of paedophile hunters were legal.
"A high court judge ruled that groups such as ours were not working outside the law and should be allowed to continue," she said.
"The letter highlights concerns they have about us jeopardising ongoing investigations, however, the cases we have brought to the police's attention have not been on the police radar for child sexual exploitation offences.
"We work hard and follow guidelines to ensure that there is no entrapment and no inciting or encouragement of child sexual exploitation."
She also highlighted the successes of the group and others in helping to bring offenders to justice.
Richard Sladdin, a former county cricket player from Calderdale, was jailed for child grooming last month after evidence gathered by a paedophile hunting group was given to police.
And a sex offender from Leeds was jailed after being caught in an online sting by a member of the Dark Justice group who posed as a schoolgirl.
Concerns about child sex abuse can be reported to police on 101, Crimestoppers anonymously on 0800 555 111 or online to CEOP here.