In a statement published on its website, the Farc leadership said it was "not its policy to threaten social or trade union leaders".

Its statement came just hours after Colombian President Juan Manuel Santos warned that Farc involvement in the threats could derail the peace talks.

The letter declared the head of a trade union a legitimate military target.

President of the General Confederation of Trade Unions of Colombia (CGT) Julio Roberto Gomez said he had received the threat last week.

The letter, signed "urban group Antonio Narino of the Farc", accused the CGT of having "sold out" and of aligning itself with right-wing forces.

Mr Gomez said his trade union, one of the largest in Colombia, had received threats by the Farc in the past.

President Juan Manuel Santos condemned the threat on Sunday, but said its authenticity needed to be verified.

"I want to believe that this pamphlet is apocryphal," he told journalists.

"It would not make sense at this stage for the Farc to declare those who are fighting for the rights of workers as legitimate targets."

He added that if the document was proven to be legitimate "it would certainly threaten the peace process".

The Colombian government and a delegation of Farc rebels have been holding landmark peace negotiations in Cuba for the past six months.

Last week, they reached agreement on land reform, the first of six major issues under debate at the talks.

The Farc leadership has strongly rejected being behind the threats against the CGT.

"No group or division of our organisation anywhere in our fatherland has made death threats against Julio Roberto Gomez or any other CGT leader," the statement says.

The Farc said the threats were the creation of "militaristic right-wing forces and police intelligence which are always sowing the seeds of discord to serve their lowly interests".

"To lend credibility to this apocryphal pamphlet is to question our sincere willingness to tread the difficult road which will lead us out of war," the Farc leadership said.

The next round of peace negotiations is due to start on 11 June in Cuba.