Colombia has ended the involvement of Venezuelan President Hugo Chavez in attempts to negotiate a prisoner swap with left-wing Colombian rebels.
Hugo Chavez has been involved in mediation since August
A Colombian presidential spokesman said Mr Chavez had broken an agreement not to speak directly to Colombia's army chiefs about the hostages issue.
Families of the hostages have expressed their shock at developments.
On Monday, Colombia gave Mr Chavez until 31 December to broker a deal on a swap of hostages for rebel inmates.
BBC Americas analyst Will Grant says Colombia's announcement comes after several days of growing frustration in Bogota at President Chavez and his apparent disregard for the proper diplomatic channels on the hostage issue.
Mr Chavez - himself a left-winger - has met Farc rebel leaders, who hold hundreds of hostages in the Colombian jungle.
Sen Cordoba (right) was involved in a series of talks with rebels
He had been invited along with Colombian opposition Senator Piedad Cordoba to mediate in the hostage crisis.
The office of President Alvaro Uribe said in a statement that Sen Cordoba had telephoned Colombian army chief, Gen Mario Montoya, and then passed the phone to Mr Chavez.
The statement said Mr Uribe previously had made it clear to Mr Chavez that he was opposed to the Venezuelan leader being in direct communication with Colombia's military chiefs.
As a result, the facilitation efforts of Sen Cordoba and President Chavez had been "terminated".
Our analyst says that although it was thought Mr Chavez's left-wing credentials might hold some real sway with the guerrilla group, many commentators felt the 31 December deadline was unrealistic.
The Revolutionary Armed Forces of Colombia, or Farc, wants to exchange about 50 high-profile hostages it is holding in exchange for 500 prisoners in government hands.
The Farc hostages include three US contractors and French-Colombian politician Ingrid Betancourt, who was kidnapped five years ago.
Her supporters have expressed their dismay at the decision to end Mr Chavez's mediation efforts.
"It is clear that the Colombian president does not want to hear about a humanitarian accord," Ms Betancourt's ex-husband Fabrice Delloye told French radio.
The Betancourt support committee called on French President Nicolas Sarkozy, who has made securing her a release a priority, to intervene immediately with Mr Uribe to ensure mediation efforts continue.
Mr Sarkozy's office on Thursday said that he would ask the French ambassador to Bogota "to deliver a letter to President Uribe asking him to maintain a dialogue with Hugo Chavez," the French news agency AFP reported.
Hugo Chavez met Ms Betancourt's children in Paris this week
Mr Chavez, who met Mr Sarkozy in Paris on Tuesday, was unable as he had hoped to provide proof that she was still alive.
But he said the Farc leader had promised to provide evidence that the hostages were alive before the end of the year.
The decision to end his mediation efforts is likely to be a severe setback for any talks between the guerrillas and Mr Uribe, whose father was killed by Farc rebels nearly two decades ago, analysts say.
"It is a sad day for the families of the hostages. We had always asked for total discretion to accomplish the humanitarian exchange," Marleny Orjuela, spokeswoman for the families of kidnapped soldiers and police officers, told Colombia's Caracol radio.