The defence secretary acknowledged that Britain was taking a greater share of the burden in frontline areas than other Nato allies, but argued that the UK had no alternative but to take responsibility.
He said: "We must be realistic about how many nations have the ability to take on the tasks facing Nato in the south and the east."
Browne insisted he would continue to lobby other Nato states to provide more troops but said: "It is increasingly clear that at present, when it comes to the most demanding tasks in the more challenging parts of Afghanistan, only we and a small number of key allies are prepared to step forward.
"This is why we have decided to commit additional forces to Afghanistan. Put simply, the alternative is unacceptable and would place too great a risk on the progress we have made so far."
Nearly 50 British troops have been killed in Afghanistan since the US-led war was launched in October 2001, many since last summer when Britain took over Nato command, spearheading a push into the volatile south.