She voiced strong criticism of a secret trial of suspected coup plotters, including the former head of state General Olusegun Obasanjo, saying the proceedings lacked judicial legitimacy.
Nigerian newspapers reported that some of the defendants have already been sentenced to death and that Gen Obasanjo has been given a 25-year jail term. But Lady Chalker said she believed no sentences had yet been passed. "I hope and pray they are premature," she said of the reports.
Speaking to diplomatic correspondents, Lady Chalker said it was "becoming quite clear that Nigeria cannot play a full role in the Commonwealth until it puts its house in order".
She said it was not for Britain to pronounce on whether the Commonwealth should expel the Nigerian government for its conduct. That, she said, was a matter for all the Commonwealth's members, who are due to meet in Auckland in November. However, she added: "It is difficult to see that General Abacha would be welcome there." Only "a rapid and credible timetable for the transition to civilian rule" could ensure otherwise, she said.
Britain believes that Nigeria must improve human rights, end the suspension of habeas corpus, end detention without trial and lift restrictions on the press, she said.
General Abacha last week lifted a ban on political activities and said a programme for a return to civilian rule would be announced at the beginning of October. But Britain is waiting to see whether the military authorities live up to their promises.
The British High Commissioner in Lagos, Thorold Nasefield, has been instructed to convey British concerns to the military government but has not always been able to gain access to General Abacha.
Britain is now consulting with its European Union partners and with fellow members of the Commonwealth about possible new measures against Nigeria. Aid has already been reduced, military training stopped and visa restrictions imposed on members of the Nigerian armed forces.
The secret trial of General Obasanjo, and other alleged plotters, has focused foreign attention anew on the political crisis that has prevailed in Nigeria, Africa's most populous nation, for the last two years.
The previous government annulled a presidential election in 1993 that was meant to end a decade of military rule. General Abacha seized power during a period of turbulence after the annulment of the vote, which was generally believed to have been won by a businessman turned politician, Moshood Abiola. Mr Abiola later proclaimed himself president but was promptly placed under detention by the authorities.
Pro-democracy groups in Nigeria claim that the government brought opponents before the secret tribunal in an effort to stifle the opposition, which is pressing for an end to military rule. The trial opened on 5 June when 23 servicemen and civilians were arraigned on charges of plotting to topple the government in a coup.Reuse content