Marines have been patrolling Kabul
The Ministry of Defence (MoD) said 300 military personnel flew from the UK on Wednesday, adding to the 200 British "enablers" and headquarters personnel already in Kabul.
The team's main task is to ensure the airport at Bagram, near the Afghan capital, is ready for the international security force when it arrives.
The landing strip at Bagram was badly damaged by air strikes at the start of the United States' military campaign in Afghanistan.
The MoD said the repair work on the airport to be done by the new influx of British personnel would "greatly ease the deployment and sustainment of the main body" of the International Security Assistance Force (Isaf).
Isaf is expected to number up to 6,000 troops in all which will be led, initially, by a British contingent of around 1,500.
Christmas patrols
The new deployment was drawn from Headquarters 3 (UK) Division and Royal Marines of 40 Commando - the same units which provided the 200 British troops already in Kabul.
The troops have been carrying out duties including clearing landmines and beginning the rebuilding of the country's shattered infrastructure.
They spent Christmas Day patrolling the streets and guarding military installations, taking a little time out for festive celebrations.
Royal Marine commandos were also used to protect the interior ministry in Kabul as Hamid Karzai was sworn in as Afghanistan's new leader.
The MoD has also confirmed that on Friday the UK would host a further planning conference of senior military personnel from the countries which will contribute to Isaf.
Details of which countries will contribute what forces have yet to be finalised.
France, Germany and Canada are all expected to send personnel to serve alongside the British forces.