The Popular Party (PP) government is planning to change the composition of the Constitutional Court, which currently has a majority of judges considered to be politically progressive, before the summer. The timing is likely to be significant because the tribunal is scheduled to review key legislation passed in recent months, and to address the issue of Catalonia's pro-sovereignty drive. The PP needs support from the court to stop a popular referendum from taking place in 2014, as announced by the nationalist Catalan government of Artur Mas.

The make-up of Spain's top judicial bodies has set the country's main parties at loggerheads on several occasions in recent years.

As things stand, on June 4 four Constitutional Court justices are set to be replaced: Pascual Sala, Ramón Rodríguez Arribas, Manuel Aragón and Pablo Pérez Tremps. The first two appointments are made by the General Council of the Judiciary (CGPJ), the judges' oversight body, with the government taking responsibility for the other two nominations.

The court currently has a 7-to-5 majority of progressive justices, resulting in several recent blows to PP appeals against issues such as same-sex marriage legislation. The court ruled that the law allowing for homosexual unions as promoted by the former Socialist government of José Luis Rodríguez Zapatero was constitutional.

Pascual Sala was proposed by the progressive majority in the CGPJ, while Pérez Tremps and Aragón were put forward by the past Socialist government. However, Aragón has ruled against the progressive majority on two occasions.

When the new justices take their place, the court will theoretically have a 7-to-5 conservative majority.