On emerging on Monday from the Central Department of Penal Investigation and Action in Lisbon, where he was grilled, the former Socialist prime minister said that he fully expected prosecutors to seek a further extension of the deadline for them to lodge charges in the case, saying that they had not yet presented him with "any proof" in 44 months of the investigation.

"It wouldn't surprise be at all if they delayed again," said Sócrates. He described as "absolutely scandalous" that prosecutors "can fail to respect the deadlines" for the case that "are in the rules that are the same for everyone".

The rules relating to deadlines for criminal investigations are indicative only, with prosecutors required to present to the attorney general their reasons for needing more time if they feel this necessary. The current deadline is Friday.

But according to Sócrates, prosecutors "have no evidence, no elements, nor facts, nor proof", only an "absurd theory" that takes the shape of "false and unjust" suspicions of corruption, tax fraud and money laundering - the crimes of which he is suspected.

"Prosecutors were not able to present anything except a phone tap here, a phone tap there, as if that could serve as a basis for any suspicion," the former prime minister said, alleging that those running the investigation are seeking to do so via the media.

Sócrates dismissed allegations that he was involved in business deals at Portugal Telecom, the partially privatised telecommunications company whose former chief executive, Zainal Bava, was named as a suspect in the case last week, to favour Banco Espírito Santo. The only intervention he made in PT, through the state's 'golden share' in the company, Sócrates said, was one that collided with BES's interests and those of its then chairman, Ricardo Salgado - also now a suspect in Operation Marquês.

"The deal to merge [PT with Brazil's] Oí was done much later, in 2013," Sócrates said, suggesting that journalists seek answers from the government of Pedro Passos Coelho, which succeeded his in 2011, and "which took the decision to give up the golden share."

Prosecutors' questioning of Sócrates for a third time took place a few days after they questioned Carlos Santos Silva, a businessman and long-time friend of the former prime minister and whom they are reported to see as a front man and recipient of millions of euros in bribes aimed at Sócrates himself. He is also a suspect in the case.

Sócrates ex-wife, Sofia Fava, who is not a suspect, is to be questioned on Wednesday while Joaquim Barroca, a director of the Lena group, which like him has been named as a suspect in the case, on Thursday.

There are currently a total of 25 suspects - 19 individuals and six companies - in Operation Marquês. Aside from those mentioned above, the other individual suspects are Armando Vara, a former Socialist government minister and former director of state bank Caixa Geral de Depósitos, João Perna, Sócrates former chauffeur, Paulo Lalanda de Castro, former director of pharmaceuticals company Octapharma, Henrique Granadeiro, former chairman of PT, Gonçalo Trindade Ferreira, a lawyer, Diogo Gaspar Ferreira and Rui Mão de Ferro, both Portuguese businessmen, and Hélder Bataglia, a Portuguese-Angolan businessman.