"The environmental impact assessment that was presented is being analysed by APA [Portugal's environment agency] and now is not a time for a political decision but a time for technical evaluation," Costa said at a parliamentary debate.

Costa was speaking in response to the member of parliament André Silva, from the People-Animal-Nature Party, who asked if the prime minister was willing to "reconsider the location of the airport," adding that the city of Beja, in southern Portugal, could be an alternative.

"Are you really going to insist on works that will damage people's health and which will devastate the avifauna of the Tagus estuary, an ecosystem that is already quite fragile?" he said.

Silva also pointed out that an assessment of the environmental impact of the Montijo airport had already been carried out and it identified "quite negative impacts for the lives of people in the region and for the population of birds that inhabit the Tagus estuary."

France's Vinci has recently agreed to provide €1.15 billion in funding for the expansion of Lisbon's main airport and the construction of a new hub in nearby Montijo, which has been strongly opposed by environmental groups.