One of Washington's most influential senators has warned Libya's opposition administration that they risk alienating US and international support unless they take "decisive action" to halt human rights abuses by rebel fighters.

The Republican Senator John McCain's stark message emerged the day after the British Government recognised the Transitional National Council based in Benghazi as the legitimate representatives of Libya and ordered the Tripoli regime's diplomats to leave the UK.

Yesterday the opposition sought to take over the Libyan embassy and consulate in London, and declared that Mahmoud Nacua, described as a 74-year-old poet, will become the new ambassador. TNC officials also urged that billions of pounds in frozen Libyan assets abroad should be turned over to the Benghazi administration.

In a letter to Mahmoud Jibril, the TNC's head of foreign affairs, seen by The Independent, Senator McCain stated: "I urge you to investigate recently documented abuses, hold people accountable as necessary, and ensure that opposition military forces are abiding by the principles of justice and human rights."

The former presidential candidate went on to stress: "As you surely know, the critics of the TNC, both in the United States and across the world, are eager to seize on any transgression to stoke opposition to the Council and to the Libyan people's fight for freedom."

Senior British Tories have long been close to Mr McCain and David Cameron, then leader of the opposition, broke diplomatic convention by backing the Senator in the presidential race in a fulsome speech in 2008.

While Mr McCain has remained a supporter of military operations, President Barack Obama is facing rising criticism from both Democrats and Republicans in Congress over the four-month long bombing campaign which has so far failed to remove Colonel Gaddafi from power.

In his letter to the TNC, dated 20th July, Senator McCain, writing as "your friend and supporter" pointed out "recent documentation of human rights abuses committed by opposition figures in the western Libyan towns of al-Awaniya, Rayayinah, Zawiyat al-Bagul, and al-Qawalish". He continued: " According to Human Rights Watch, a highly credible international non-governmental organisation, rebel fighters and supporters have damaged property, burned some homes, looted from hospitals, homes and shops, and beaten some individuals alleged to have supported government forces.

"I am confident you are aware of these allegations.... It is because the TNC holds itself to such high democratic standards that it is necessary for you and the Council to take decisive action to bring any human rights abuses to an immediate halt."

The rebel forces, who have failed to achieve any game-changing military successes despite the air strikes destroying much of the regime's capabilities, have also been accused of being divided and sending contradictory and conflicting policy statements.

Disarray among opposition ranks was illustrated when the TNC's co-ordinator in Britain, Guma el-Gamaty, dismissed as "silly" the recent offer by the TNC head, Mustafa Abdel Jalil, that Colonel Gaddafi and his family could stay in the country rather then go into exile. "I think we are coming to the end of all these silly political initiatives and all this talk about Gaddafi staying in Libya," he said. Mr Gamaty's stance also runs contrary to the position of the French government that peace talks can begin with Colonel Gaddafi staying in the country as long as he relinquishes power. The Foreign Secretary, William Hague, has also endorsed this position although he has stressed that it was ultimately a question for the Libyan people to decide.

Dear Dr Jibril...

* "You have no greater friend and supporter in the United States Congress than me, and I will continue working tirelessly to urge my government to take further steps that would support your efforts... It is as your friend and supporter that I write to you regarding the recent documentation of human rights abuses committed by opposition fighters..."

* "I am confident you are aware of these allegations, and I know that the principles espoused by the TNC [Transitional National Council] stand in stark and positive contrast to the acts of cruelty that the Gaddafi regime continues to perpetrate against the Libyan people. It is because the TNC holds itself to such high democratic standards that it is necessary for you and the Council to take decisive action to bring any human rights abuses to an immediate halt."

* "It is equally important for the TNC to make clear through public statements that such acts will not be tolerated and that the TNC remains fully committed to the protection of human rights. I urge you to investigate the recently documented abuses, hold people accountable as necessary, and ensure that opposition military forces are abiding by the principles of justice and human rights that the TNC has correctly championed."