Boris Johnson will hold an emergency Cobra meeting this afternoon to "discuss the current situation in Afghanistan'', No 10 has announced.
It comes as Britain and the US have agreed to send more troops in as the Taliban's lightning advance through the country continues - gains that come only weeks before allied forces are due to withdraw.
Taliban insurgents are now estimated to hold more than two-thirds of Afghanistan and continue to press their offensive, having taken the country's second and third largest cities, Kandahar and Herat, as part of a week-long blitz.
Afghan officials announced on Friday that the Taliban had captured Lashkar Gah, the capital of the southern Helmand province.
Cabinet minister Ben Wallace denied the plan to send 600 troops in to Afghanistan, announced on Thursday, was a last-minute decision, arguing that it was put in place "some months ago" in preparation for withdrawal alongside the US by September 11.
The short-term deployment, which comes as the US vowed to send 3,000 of its troops to Afghanistan, will be used to support diplomacy, help citizens leave the country and support the relocation of former Afghan staff as the Taliban continues to make in-roads.
Mr Wallace, who refused to rule out further ground attacks or air strikes if the situation worsens, admitted he is concerned multi-national terror network al Qaida, the group behind atrocities such as the 9/11 attack on the World Trade Centre in New York, could "come back" as Afghanistan de-stabilises once again.
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