The UK is to deploy 1,400 more troops to Afghanistan, bringing numbers up to 7,700, Des Browne has told MPs.
The majority will be deployed over summer and will be mainly based in the volatile province of Helmand, where UK forces have been fighting the Taleban.
The defence secretary said the move was being made with the agreement of the chiefs of staff and was "manageable".
He denied that the need for more forces in Afghanistan had forced the recent decision to reduce troops in Iraq.
And, in a Commons statement, he said it would be an "error" to believe that the need to increase troop numbers was the result of poor planning.
Nato commanders have said they need more soldiers to tackle an expected spring offensive by the Taleban.
The UK says it is having to send more troops because of the reluctance of fellow Nato members' to send their forces to southern Afghanistan.
Mr Browne said: "We believe every Nato partner should be prepared to do more to meet this need. But we must be realistic.
Battle group will include elements from:
1st Battalion Royal Welsh Fusiliers
1st Battalion Scots Guards
5th Regiment Royal Artillery
39th Regiment, Royal Artillery
Source: Ministry of Defence
"I have lobbied our partners consistently for more help but it is increasingly obvious that at present, when it comes to the most challenging parts of Afghanistan, only we and a small number of key allies are prepared to step forward."
He added that the increase in troops was done "in accordance with unequivocal military advice".
Some of the forces deployed will be reservists although numbers are yet to be confirmed. The troops will be committed until 2009, Mr Browne said.
As well as Helmand, the troops will also be operating in the neighbouring province of Kandahar, and the provinces of Uruzgan, Zabul, Nimruz and Day Kondi.
The announcement of the extra UK deployment comes days after Prime Minister Tony Blair said the country's military presence in Iraq was being reduced by 1,600 personnel.
British troops will continue to be based in the south
For the Conservatives the shadow defence secretary Liam Fox said the increase indicated that "we are taking a disproportionate burden".
He described it as "scandalous that only four Nato nations ... the UK, the US, the Canadians and the Dutch - surprise, surprise - are contributing by far the greatest to the security in the south, and the most dangerous parts of the country.
"The UK taxpayers and the UK military are taking far more of the share of the burden than we should in what is supposedly a communal operation."
He said success in Afghanistan was "essential" for global security and said there were now questions to be asked about the future of Nato.
Liberal Democrat defence spokesman Nick Harvey said it was frustrating that Nato allies were not participating more, but he understood the "logic" of the increased deployment.
He questioned defence planning, saying "if we are going to keep up the operation in Afghanistan over a period of years, isn't it essential we complete the draw-down from Iraq as quickly as possible so that we don't continue operations on two fronts."
Mr Browne's statement came as Foreign Secretary Margaret Beckett visits Pakistan for talks with President Pervez Musharraf.
She said: "I welcome the opportunity to discuss the work we are doing together on counter-terrorism, as well as other aspects of our relationship.
"I also expect to discuss the steps we are taking to defeat the Taleban in Afghanistan. Pakistan is a vital partner in this region and their commitment is essential to creating a stable and secure Afghanistan."