The M25 must be closed when protesters climb gantries because “our number one imperative is safety”, National Highways said.
Tens of thousands of drivers suffered long delays this week as Just Stop Oil activists repeatedly carried out protests on the M25.
Many motorists are using social media during the demonstrations to urge authorities not to close the UK’s busiest motorway.
But Sean Martell, who is leading National Highways’ response, told the PA news agency that is not an option.
He said: “There is a risk of (protesters) falling from the gantry.
“There is a risk of items falling from the gantry that might be in their possession.
“Our approach has been to work with the police to initially close the road and assess the risk that that person on the gantry poses.
“Where possible we’ll consider what lanes we can reopen, but our number one imperative is safety and we have to think about the safety of all road users when these protests happen.”
He said closures are usually for about two hours, but some have lasted up to four hours, creating major disruption.
Asked if anything is being done to the gantries to make it harder for activists to climb them, Mr Martell said: “They are designed for maintenance so there has to be a degree of accessibility to our maintenance team.
“We check the security of the gantries regularly, and in some cases during these protests we’ve seen evidence that the security has been tampered with.”
Dealing with Just Stop Oil is costing National Highways “a lot of money”, Mr Martell said.
“That’s a resource that we want to use to do our primary role which is connecting the country and ensuring that people can get around the network safely.”
He added that National Highways is “working very hard in dealing with protesters from a legal perspective”.
The organisation secured a High Court injunction last week which means anyone entering the M25 and fixing themselves to any object or structure on it, and anyone assisting in such an act, can be held in contempt of court.
They could face imprisonment, an unlimited fine and the seizure of assets.
The injunction was secured in addition to a court order obtained by National Highways earlier this year that targeted protesters including those from Insulate Britain.