The head of state, Marcelo Rebelo de Sousa, vetoed the bill - which introduced changes to an existing statute on the transfer of the city's bus and tram, Carris, to the city council - arguing that it was wrong to prohibit any future transport concession of this kind.

Speaking in parliament, Socialist deputy Pedro Delgado Alves said that the government would try to "find a way of making compatible the concerns that underlie this bill with the concerns of the President of the Republic."

The Socialist Party view, he said, is that "it is up to Lisbon council to take the structural decisions on the company and on its role in the management of the city", noting that "on that aspect there is no objection presented" by the president.

“We shall contribute to a balanced solution that takes into account the concerns that are on the table," he said.

The president has previously rejected similar legislation on Porto's metro operator and bus and tram company.

De Sousa has exercised his veto of bills without formally questioning their constitutionality by sending them to the Constitutional Court for review a total of five times since he took office in March last year.

Some other politicians supportive of the minority Socialist government were more critical of the president, with the Left Bloc candidate for Lisbon mayor, Ricardo Robles, saying that the veto was “clearly political” and resulted from the “agenda of the right” - that is, to block a transfer of control of transport in Lisbon from central to local government under certain conditions.

The right-of-centre Social Democratic Party that de Sousa once led was by contrast in agreement with his move. Its deputy leader in parliament, Luís Leite Ramos, said that he was right to veto “excessive” changes to the statute in question, in the form of legislation that represented an “attempt to condition the ... exercise of municipal autonomy, by imposing a management model on the Lisbon council relative to the management of Carris”.

Opposition politicians have accused the left of wanting to retain the transport companies in the public sphere because their heavily unionised workforce are a bastion of power for left parties.