A UN panel has criticised Colombia after a study found state officials played a role in a large majority of the 1,161 disappearances since 1980.
The UN says people are often too afraid to report disappearances
It urged Colombian President Alvaro Uribe not to ratify a new amnesty law for demobilised fighters.
Human rights groups say the law offers the perpetrators of crimes concessions without ensuring they help inquiries.
Mr Uribe is currently on a trip to Europe seeking support for Colombia's ongoing peace process.
He met UK Prime Minister Tony Blair in London on Thursday to discuss efforts to tackle Colombia's illicit drugs trade and levels of violence in the country.
Colombia's Congress passed the Justice and Peace Law, a bill setting out the rules for disbanding paramilitary and rebel armed groups, last month.
The legislation, which offers reduced prison sentences and financial benefits for those demobilised, is still to be approved by Mr Uribe.
Santiago Corcuera, of the UN Working Group on Enforced or Involuntary Disappearances, said the situation regarding disappearances in Colombia remained "serious".
Paramilitaries or armed forces on the margins of the law were to blame for the majority of cases, with the support or complicity of agents of the state, Mr Corcuera said.
Speaking at a press conference, he said dozens of new cases had been reported to the panel during the course of its week-long investigation.
He said victims' families had previously often been too scared to testify or even to report the disappearances.
Colombia's armed forces have often co-operated with the right-wing paramilitary groups against rebel forces, their common foe.
Mr Corcuera said the failure of Colombia to bring the perpetrators of disappearances to justice meant the victims were seldom found, whether alive or dead.
Rights groups, including Amnesty International, are concerned the new legislation goes too far to protect paramilitary criminals.
In London, Amnesty secretary-general Irene Khan appealed to Mr Uribe not to ratify the "deeply flawed" law on human rights grounds.
She said the law failed to respect victims' right to truth, justice and reparation.