DNA tests on a young boy in a Colombian foster home suggest that he is probably the son of a rebel-held hostage, the country's chief prosecutor has said.

Correspondents say the findings are a big blow to the credibility of the Farc rebel movement, which claimed to be holding the boy with his mother.

Questions over his identity emerged amid a Venezuelan-led mission to oversee the release of the pair.

Rebels had said the release was delayed because of army operations in the area.

The Revolutionary Armed Forces of Colombia (Farc) announced last month that they were prepared to release Clara Rojas and her son Emmanuel, as well as a former congresswoman, Consuelo Gonzalez de Perdomo, as a goodwill gesture.

Emmanuel, said to be around three years old, is believed to have been fathered by one of Ms Rojas' captors.

Ms Rojas was kidnapped in 2002 along with former presidential candidate Ingrid Betancourt.

Venezuelan complaint

Colombian authorities say a malnourished boy matching Emmanuel's description was brought to a hospital in San Jose del Guaviare in June 2005 and then wound up in a Bogota foster home, his real identity unknown.

The tests indicated a complete match between mitochondria in the blood of the boy - who has been given the name Juan David Gomez - and Ms Rojas' mother, Chief Prosecutor Mario Iguaran said, quoted by the Associated Press news agency.

This meant a "very high probability" that he was Ms Rojas' son, he added.

Mr Iguaran said it would take up to two weeks for a European laboratory to confirm the DNA analysis.

The BBC's Jeremy McDermott in Colombia says the test results show the Farc rebels were either lying or incompetent, and the news is a severe blow to their credibility.

However, Venezuelan Foreign Minister Nicolas Maduro said the failure to allow Venezuelan specialists to take blood samples from the boy cast a "cloak of doubt" on the results.

Venezuelan President Hugo Chavez, who was involved in mediation efforts, had put together an international mission to oversee the handover of the hostages.

But on Monday Mr Chavez read out what he said was a letter from the Farc on Venezuelan television.

The rebels said that continuing operations by the Colombian army had prevented them from turning over the hostages.

Little trust

Our correspondent says the test results are unlikely to change the plight of hostages in Farc hands.

They are unlikely to make any concessions and continue with their demands that hundreds of imprisoned rebels be freed in exchange for their hostages, he adds.

The Farc hold some 45 high-profile hostages, including Ms Rojas, who are considered "exchangeable".

But it is believed the guerrillas hold about 750 hostages in total, some for political leverage but many for ransom.