Tony Blair wants a "flexible" Nato force to prevent Afghanistan falling back into the hands of the Taleban.
Tony Blair wants other Nato nations to commit more troops
He is joining other Nato leaders in the Latvian capital Riga for a summit to be dominated by events in Afghanistan.
Mr Blair, President George Bush and Nato commanders are pressing for more deployments in the fight against the Taleban in the south of the country.
But member states including France, Germany, Italy and Spain, have limited their troops to more peaceful areas.
Speaking ahead of a Nato meeting in Latvia, Mr Bush berated Nato members reluctant to send troops to Afghan hotspots, demanding they must accept "difficult assignments".
Last week, Mr Blair reiterated the UK's support for the Nato mission and the Afghan government during a visit to the country.
En route to Riga, Mr Blair said during his visit he found - "contrary to some of the things I expected" - determination amongst British forces "but also among the Afghan authorities to make sure that the Taleban were not allowed back into Afghanistan and the country was not again to become a breeding ground for terrorism".
Outlining what he hoped to gain from the Nato summit, Mr Blair said there were four requirements, including military and government roles.
He said it was important to have "additional flexibility and force generation" to make the mission a success.
He emphasised the reconstruction and development was essential in showing the Afghan people that progress was being made.
"And then finally, of course, it's about building up the capability of the Afghan government."
He also wanted the mission to be restated "with confidence".
There are about 32,000 Nato troops in Afghanistan, with the majority of the 6,000 British based in the southern Helmand province which has seen some of the heaviest fighting.
The UK, US, Canada, and the Netherlands are expected to press other member states to commit more troops.
Tory defence spokesman Liam Fox, in a speech to the Conservative Way Forward group, criticised Germany, Italy and Spain for limiting troop activities to the relatively peaceful west and north of the country.
Mr Fox said the British, Canadians, Australians, Americans and "a few honourable others" were operating as a single Nato force, while the Germans, Italians and Spanish saw themselves as national forces under a Nato umbrella.