Portuguese President Jorge Sampaio has called an early general election on 20 February, just four months after the centre-right government took office.
The government of Mr Lopes (left) has seen falling ratings
Mr Sampaio said that dissolving parliament was the only way to solve a crisis of credibility and instability.
Prime Minister Pedro Santana Lopes took office when Jose Manuel Barroso become president of the European Commission.
Their Social Democratic Party is currently trailing in the opinion polls behind the opposition Socialists.
"This government's negative succession of incidents and
declarations, contradictions and discoordinations in whole led me to dissolve the parliament as the only solution for a grave crisis of credibility and instability," Mr Sampaio said in a televised address.
The announcement came 11 days after Mr Sampaio had said he intended to dissolve parliament and call an early vote.
The brief tenure of Conservative Prime Minister Santa Lopes has been marked by the bitter resignation of a minister, slumping polls and allegations of government interference with the media.
The Socialists, under the new leader Jose Socrates, are believed to have a good chance of winning the February election.
The Social Democratic Party and Popular Party are to continue as a caretaker government until the election.