Foday Sankoh, leader of the Revolutionary United Front (RUF), said in a radio message that the two aid workers were disguised military advisers.
Mr Sankoh referred to the court martial of Lieutenant-Colonel Chernor Deen, who was sentenced to death by firing squad on Wednesday and said that if he is executed, the Britons being held by the RUF will die.
Robert D'Cruz, aged 30, and Calum Murray, 25, were seized on 7 November at Kabala, 155 miles north-east of the capital, where they were working on a project for the British charity Voluntary Service Overseas.
The Foreign Office said the two Britons had nothing to do with the former British colony's military government. "The safety of the hostages is the responsibility of the RUF. We have made sustained efforts to negotiate their safe release, but the RUF has shown no willingness to negotiate," a Foreign Office spokesman said.
Mr Sankoh ended his two-hour address by stating he would call again at 1100 GMT on Saturday and advised the government to consider his demands by then.
Officials said there had been no contact with the RUF since 16 December despite efforts by negotiators to contact them.
The British aid agency said: "From the beginning of the situation we have consistently urged the government of Sierra Leone to avoid any action, which might endanger the lives of our volunteers."
Mr Sankoh's forces began a guerrilla war in the south and east of the country in May 1991 against the regime of President Joseph Momoh, who was ousted in a coup d'etat the following April.
The military government recently opened dialogue with the RUF despite having continued the war since the coup. The junta has pledged a return to democracy in January 1996.