Security Correspondent, BBC News
The government is to send up to 35 extra diplomatic staff to Afghanistan, the BBC has learned.
Taleban insurgents are becoming more resilient
The deployment will make the country one of the Foreign Office's biggest overseas postings.
Whitehall sources said the move is an attempt to prevent the country suffering the same level of chaos and violence as Iraq.
Officials said staff will focus on tackling drug production and corruption as well as building institutions.
Currently there are between 50 and 100 UK-based diplomats in Afghanistan, including counter-narcotics specialists.
The new staff are expected to be deployed to the British Embassy in Kabul and to Lashkar Garh in the south over the coming months.
Foreign Office officials say the priorities will be to combat corruption, help build government institutions in the south and to tackle the production of opium.
The newly enlarged embassy staff will be headed by one of Britain's highest profile diplomats, Sir Sherard Cowper-Coles, who is currently ambassador to Saudi Arabia.
A fluent Arabist, he was previously ambassador in Tel Aviv where he learnt to speak Hebrew.
The increased diplomatic presence comes at a time when Taleban insurgents are proving to be a more resilient and dangerous enemy than many in the West had expected.
In April, Britain will take over command of Nato forces in the south from the Canadians, giving British commanders overall responsibility for some of the most violent provinces in Afghanistan.
There are currently close to 6,000 British troops in the country, mostly deployed in the southern province of Helmand.
The British government had hoped that its mission in Afghanistan would be primarily aimed at stabilisation and humanitarian assistance.
Only last year the defence secretary at the time, John Reid, was quoted as saying that it would be good if British forces could complete their mission in the south without a shot being fired in anger.
But the Taleban have done exactly what they said they would: resist what they see as a foreign army of occupation.
Using bases and popular tribal support on the Pakistani side of the border the resurgent Taleban have been able to mount frequent attacks on Nato forces despite suffering heavy casualties themselves.
They have carried out a campaign of intimidation in parts of the south and east, warning the local population not to cooperate with the elected government of President Hamid Karzai.
They have also burned down dozens of schools, threatened aid workers and killed Afghan school teachers.
Although British and Nato forces have been winning almost every fire fight at the tactical level, partly thanks to their ability to call in close air support, their battle to win over Afghan hearts and minds has not been going so well.
Many of the clashes result in the wholesale destruction of local property and some civilian loss of life.
Nato forces have also not been helped by a return to widespread corruption and banditry in rural areas, an endemic problem which helped usher in the reign of the Taleban with their draconian punishments in the 1990s.
The Taleban have also been vigorous in spreading their message to the Afghan population in the south and east that they are here to stay while Nato's presence will only be temporary.