In comments to Lusa the party’s parliamentary leader, Carlos César, said that the PS has "no intention” of voting in favour of the BE proposal.

The levy’s proponents had argued that the levy – which would be imposed on capital gains from the sale of property bought relatively recently – is needed stop speculation from squeezing out affordable housing in Lisbon and Porto in particular. But César rejected the idea.

"On the contrary, speculation can’t be fought with a levy that is a repetition of the tax on capital gains that already exists,” he said.

“Speculation is fought effectively with an increase in the amount of affordable housing on offer, as the government has proposed [in legislation that] awaits approval in parliament.”

On Sunday, the BE's coordinator, Catarina Martins, unveiled a proposal she said was aimed at curbing property speculation, arguing that the measure could be be approved as part of the 2019 state budget, which the PS government is currently drafting.

Martins said that the proposed levy would be similar to those proposed for "speculative movements on the stockmarket" and would consist of a special rate of tax on "large" profits made by investors who buy and sell property "in a short period of time".

The PS lacks a majority in parliament but has been able to govern since it took power at the end of 2015 thanks to agreements with the BE and other left-wing parties.