A series of Brexit-themed posters that appeared across London today have been "flyposted illegally", outdoor advertising company Clear Channel have said.
The posters, which depict Boris Johnson holding Russian flags, and another featuring Russian President Vladimir Putin winking, were pasted over existing posters on Clear Channel-owned advertising space.
The billboards have been put up by a group calling itself Proud Bear, who claim to be a collective of Russian GRU agents who want people to "celebrate their role in 2016's Brexit vote".
But according to source code hidden on their website, they are a group of anti-Brexit volunteers.
Clear Channel are now working to have the posters "removed as soon as practically possible", and are considering taking "further action" against the anonymous campaigners.
A source at Clear Channel said the brand were trying to work out "what the hell was going on", after they were first alerted to the posters.
'Removed as soon as practically possible'
In a statement, Clear Channel said: "The Proud Bear billboards have been flyposted illegally on Clear Channel sites.
"As with all cases of flyposting, we are seeking to have them removed as soon as practically possible and we are considering further action."
On their website, Proud Bear say they are raising money to have an advert displayed on a 40-metre wide LED screen at London's Waterloo train station, which costs £55,000 to rent.