Founded in 1985, the Patriotic Union was barred after it failed to present candidates to elections in 2002.

But the top administrative court has now ruled the party had been targeted by state-sponsored death squads, and decided to re-instate its status.

The news comes amid ongoing peace talks between the Farc and the government.

The landmark negotiations, aimed at ending five decades of violence, are taking place in Cuba.

The two sides have been discussing political participation for Colombia's largest guerrilla group if a peace deal gets signed.

The Patriotic Union (UP) was established during the 1985 peace talks with the government of then President Belisario Betancourt.

But the UP lost its legal status when it did not put forward candidates to the parliamentary and presidential elections in 2002.

In Tuesday's ruling, however, the State Court recognised there were mitigating circumstances for the UP not complying with Colombian electoral law, says the BBC's Arturo Wallace in Bogota.

About 3,000 of its members, including several presidential candidates, were murdered by right-wing paramilitaries and drug traffickers with the complicity of the government.

Analysts say this convinced the Farc rebels that the political route was closed to them, instead resorting to military action.

Some 600,000 people have been killed since the conflict began in the 1960s, and more than three million internally displaced by the fighting, according to official estimates.

The Farc is thought to have some 8,000 fighters, down from about 16,000 in 2001. Last year they renounced kidnapping for ransom, a policy which had made Colombia one of the countries with the highest kidnapping rate in the world.