The Spanish conservative Miguel Arias Cañete was confirmed as the EU’s new climate and energy commissioner on Wednesday after a deal between centre-left and right parties in the European Parliament, despite protests from environmentalists.

The centrist pact saw Liberal former Slovenian prime minister, Alenka Bratušek, overwhelmingly rejected by MEPs as the commission’s new energy union commissioner, but the words ‘sustainability, climate action and energy’ added to the portfolio of Cañete’s boss, the Dutch socialist, Frans Timmermans.

MEPs had demanded this caveat as a condition for supporting Cañete, a former director of two oil companies. But after more than half a million people signed an Avaaz petition calling for Cañete’s rejection, environmentalists were left fuming at a perceived democratic deficit in the EU.

“MEPs betrayed our hopes and placed a petrol head in charge of Europe’s climate policy,” said Luis Morago, Avaaz’s campaigns director. “The leaders of the Socialists and Democrats have broken their promises. They have neither listened to Europe’s voters or given people back trust in the EU. The public campaign to reject Cañete was always going to be tough because of horse trading in Brussels, but people across Europe will not stop until their representatives lead the world in action against climate change.”

Many Socialist and Democrat (S&D) MEPs were uneasy about the final agreement but cited a need to avoid the “political chaos” which would follow a rejection of Cañete, potentially involving a break-up of their nascent pact with the centre-right European Peoples Party (EPP) group.

The Belgian S&D MEP Kathleen Van Brempt told the Guardian that she understood the criticisms but that the change to the portfolio of Timmermans, a Dutch social democrat, would safeguard climate change priorities at the EU’s highest level.

“We were not giving [i.e. throwing] a party when we voted for Cañete,” she said. “We are as concerned about climate issues as most of these people. It is one of our top priorities. But this is political life, we had to compromise, and we made sure that Cañete will be supervised by the strongest possible commissioner in Juncker’s commission. Timmermans has a lot of authority and Cañete will not be able to do anything by himself. On his own he’s nothing.”

Friends of the Earth, though, said that they were disappointed at the appointment, citing a potential conflict of interests as Cañete had been replaced as chairman of the two companies by his brother-in-law, Miguel Domecq Solis.

“People will be watching his actions closely and he will have to prove he is acting independently, and working for the benefit of the climate, not polluters,” said Magda Stoczkiewicz, Friends of the Earth Europe’s director.