The letter, which has been seen by Reuters, comes just a day after a Socialist-led coalition managed to oust the centre-right government from office. The new left-wing coalition could be brought into office within days unless the Portuguese Presidents opts for a new general election and appoints a caretaker government.
“The Socialist Party does not accept that the state does not maintain a controlling stake, as it should have a direct say and presence in the strategic path of this company," the letter said.
As the centre-right is now ruling in a caretaker capacity, "the legal and political conditions are not present" for the TAP sale to go ahead, said the letter.
The sale of the airline was expected to be concluded tomorrow. The situation echoes that of Greece when the far-left Syriza party took over in January and immediately tried to halt all privatisations arranged by the previous government.
Syriza was eventually unsuccessful after agreeing to go ahead with privatisations in exchange for emergency bailout cash from the International Monetary Fund and the Eurozone. However, the Portuguese Socialist Party will be under far fewer restrictions.