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A TV helicopter filming a bike race in Spain accidently busted a marijuana rooftop farm while broadcasting the event from the air.

Catalan police officers reportedly seized over 40 cannabis plants from the roof near Barcelona after gathering information from the TV helicopter covering this month's Vuelta a Espana race.

The eighth stage of the race finished on Saturday in the town of Igualada, where cyclists could be seen making their way through the streets.

With an aerial view of the race, the TV camera zooms past a rooftop covered with dark green plants and solar panels, accidently capturing evidence of the covert weed operation.

Although it is not illegal to grow marijuana for personal use in Catalonia, it is illegal to sell it so authorities can still levy fines on large operations where the amount exceeds the legal threshold, according to local paper Ara.

The publication reported that police had already opened a case against the occupants of a flat in the building, after neighbours told officers the property had been emptied, the furniture was dumped in the street and there were marijuana plants on the roof.

After the footage emerged on the weekend, police reportedly had enough evidence to search the block of flats.

"We received information thanks to the images that individuals posted on social media that had been taken from the helicopter of La Vuelta," a police spokesman told ESPN.

"That has helped us to seize 40 marijuana plants. No one has been arrested, but the investigation is still ongoing to find those responsible.”