António Costa, speaking at an election campaign event in Setubal, said the right-of-centre coalition government was caught "once more in a trick" that he described as being "to disguise" the 2012 accounts".


"What we ask ourselves is how many tricks are yet to be discovered," he said. "It's dramatic for the credibility of political life and democracy when ... not a week goes by in which the government isn't caught in an attempt to fool citizens."


State radio had earlier reported that the finance minister, Maria Luís Albuquerque, had as secretary of state for the treasury asked the board of Parvalorem, the company set up to manage BPN's 'toxic' assets after it collapsed in 2008 and was taken over by the state, to alter the 2012 accounts so that they would show losses of less than €150 million.


In the Socialist Party's first reaction, spokesman Eduardo Cabrita demanded that the prime minister, Pedro Passos Coelho, provide urgent clarification about the process.


In response, the government rejected the charge that there had been "any manipulation or hiding of accounts", while Parvalorem described the 2012 accounts as "nothing other than normal".


In his speech later, Costa said that there are "no numbers, no tricks, no accounting that can disguise the concrete reality of people's life" such as mass emigration by young people in search of work.


The governing coalition has pulled ahead of the Socialists in some recent polls of voter intentions for the 4 October general election.