LONDON, June 20 (Reuters) - Britain needs to maintain a significant presence in Afghanistan for several decades to help fight terrorism and pull the country out of poverty, London's ambassador to Kabul said on Wednesday.

Sherard Cowper-Coles, who took up his post as ambassador six weeks ago, said Britain needed to commit to a long-term presence in Afghanistan to help it recover from 30 years of war and quash a determined Taliban insurgency.

"The task of standing up a government of Afghanistan that is sustainable is going to take a very long time," he told BBC radio. "It's a marathon, not a sprint. We should be thinking in terms of decades."

Afghanistan has relied largely on Western troops for its security and on donor funds for its economy since U.S.-led forces overthrew the Taliban in 2001.

Britain has about 7,000 troops in Afghanistan, based mainly in the southern province of Helmand where they have encountered fierce resistance from Taliban fighters. The total deployed will rise to 7,700 this year.

However, Cowper-Coles said any decades-long British presence would not necessarily be military.

"But we are serious about a long-term development presence," he said, adding Britain's foreign office had decided Afghanistan was now "one of our very highest foreign policy priorities".

Cowper-Coles rejected suggestions that U.S.-led coalition forces were losing the fight with the Taliban, and losing the support of ordinary Afghans for their presence.

More than 120 civilians have been killed in recent months during foreign troop operations, according to Afghan officials and witnesses. The deaths have sparked protests, including demands for the withdrawal of U.S.-led coalition forces.

The British ambassador admitted "mistakes have been made" with regard to the killing of civilians, but pointed to human rights organisations' data which showed the Taliban killed five times as many civilians.

"The reality is that the great majority of Afghans want us here," he said. "If there is one thing they are clear about it is that they do not want to return to the dark days of mediaeval Taliban rule."

Prime Minister Tony Blair, due to step down next week and hand over the British premiership to finance minister Gordon Brown, has warned Afghanistan risks being overwhelmed by anti-western violence, similar to that in Iraq.

Cowper-Coles said he hoped Brown's new government would remain committed to Afghanistan for the long term.

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