Scarborough Council has said it will step up security after a group filmed inside the soon-to-be demolished Futurist theatre.
Urban Explorers got inside the building over the weekend, capturing almost eight minutes of video.
Now, the council has warned those responsible to seek medical help as they may have been exposed to asbestos.
In a joint statement Scarborough Borough Council and Willmott Dixon said: " On 24 February Urban Explorers broke into the Futurist building and recorded a video of various internal areas, which was subsequently posted on the internet.
"The video shows the exceptionally poor condition of the building and that stripping out work is well advanced.
“The building had security guards present and once they became aware the trespassers were in the building, the police were notified. We are currently using the network of CCTV cameras throughout the town to attempt to identify those responsible.
“Any demolition or construction site is a particularly dangerous environment and people who intentionally trespass are putting themselves and others in great danger of death or serious injury.
"In the case of the Futurist, the building contains asbestos and other dangerous materials and hazards which could harm unauthorised persons accessing the site. It is a miracle that no one was injured although the effects from exposure to asbestos can take many years until they are known, and we would urge those responsible to seek medical attention.
“Scarborough Borough Council and Willmott Dixon have taken all reasonable measures to prevent access including hoarding, boarding up, providing security and erecting warning signs to highlight the dangers.
"We will also shortly be installing a new CCTV system to cover the whole of the site. Members of the public must not attempt to access the Futurist site now the works to demolish the building have commenced."