Josep Borrell served on the board of Spanish energy firm Abengoa between 2009 and 2016.
The company was involved in the construction of combined-cycle gas turbine power plants.
During his stint with Abengoa, Mr Borrell received up to €300,000 a year.
Three of Ursula von der Leyen s top EU officials have links to fossil fuel firms (Image: GETTY)
European Commission Presdent Ursula von der Leyen (Image: GETTY)
The Spaniard was forced to resign as the president of the European University Institute after failing to declare his interest in the firm in 2012.
Mr Borrell holds a master’s degree in oil economics and was an engineer for Spanish petrol giant Cepsa.