The UK force in southern Afghanistan will be boosted by about 900 troops with extra helicopter support.
Defence Secretary Des Browne told the Commons 320 engineers from 28 Regiment Royal Engineers would be used to "accelerate the reconstruction effort".
A company from 3 Commando Brigade Royal Marines would provide force protection for them and those deployments would take place from September.
An additional infantry company, drawn from the 2nd Battalion The Royal Regiment of Fusiliers, will also provide mobile forces and two platoons from the 1st Battalion, The Royal Irish Regiment would provide additional protection.
Mr Browne said there would also be a "small increase" in HQ staff and medical and logistical support.
The Defence Secretary insisted the force increases in the flashpoint Helmand province were because "the commanders on the ground grasped an early opportunity".
He went on: "They saw the chance to reinforce the position of the local governor and the Afghan army and police by going into northern Helmand and challenging the impunity of the Taliban there."
Mr Brown said some extra troops would be going immediately, others in October, but "as a result of today's announcement the steady-state size of the Helmand Taskforce will increase between now and October from some 3,600 personnel to some 4,500 personnel".