Monteiro, who served in the Social Democrat-led coalition government that was in power from 2011 to 2015, is to be questioned on 4 December, with the focus on his earlier involvement in the concession relating to a planned high-speed rail line.

In the Operation Marquês case files, which have been reviews by Lusa at the Lisbon court where the case is being handled, it is stated that Monteiro was involved in issuing the bank guarantee provided by Caixa BI, a unit of state bank Caixa Geral de Depósitos where he worked before being in government, under the programme relating to the competition for the concession of the Poceirão-Caia section of the planned high-speed rail link between Lisbon and Madrid, which ended up not being built.

In the file a reference is also made to clauses suggested by the Elos consortium to the banks financing the concession deal.

While working at Caixa BI, Monteiro was involved in creating the financial instruments for the consortium for the construction of the first planned section of the rail line.

Monteiro’s name was also mentioned by witnesses Paulo Campos - who served as secretary of state for transport in the previous, Socialist government led by Sócrates - Carlos Alberto Fernandes and Pedro Manuel Nascimento as being linked to the financing of the Elos consortium, among whose shareholders were Portuguese construction companies, notably the Lena group, which is also being investigated as part of the Operation Marquês case.

Ivo Rosa, the judge who is overseeing the review of the case, takes the view that it is important at this stage to hear Monteiro, since he had direct knowledge of the facts relating to the competition for the concession.

Sócrates, when interrogated during the previous, investigation phase of the case, denied having been involved or having received money for awarding the concession to build the first section of the high-speed network to the Elos consortium. Public prosecutors allege that the then prime minister intervened in favour of the Lena group and its business partners.

On Wednesday it is the turn of Carlos Santos Silva, a businessman and longtime friend of Sócrates who is also among the accused, to be interviewed by the judge and prosecutors.

The Operation Marquês case began more than five years ago. In all, 28 people have been charged on a total of 188 counts of various economic and financial crimes.