STV is in the sights of an activist investor which has snapped up a 10% stake in the broadcaster.

It is thought that Crystal Amber, which recently raised its holding in newspaper publisher Johnston Press, wants to buy as much as a fifth of STV.

Crystal Amber is said to have become “frustrated at the channel’s plummeting share price”, according to City sources.

Its share buying could put STV, led by chief executive Rob Woodward, on the radar of potential bidders.

Shares in STV are down 8% over the last 12 months but have risen 5% since the beginning of the year. They closed last week at 400p.