Germany, Italy and Spain are only sending troops to "low risk" areas in Afghanistan, while other Nato allies are on the front line, says Liam Fox.
3 Para are back in the UK after a six month tour of duty
The Tory defence spokesman criticised the three governments for limiting troop activities to the relatively peaceful west and north of the country.
In a speech to the Conservative Way Forward group, Dr Fox said the approach was "not acceptable".
He singled out three of Nato's biggest players on the eve of a summit in Riga.
In his speech, Dr Fox said the caveats put on troops by Berlin, Rome and Madrid kept them away from the fierce fighting encountered by British, Dutch and Canadian troops in the south.
'Life and limb'
He 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.
"You cannot agree to be part of a military alliance and than choose to deploy only to areas where there is a low level of risk," he said.
NATO FORCE IN AFGHANISTAN
31,000 troops now on ground in Afghanistan, including 10,000 coalition troops moved under Nato command
37 nations contributing
8,000 US-led troops continue training and counter-terrorism separate from Nato force
*Contribution figures may differ from exact numbers on the ground
"Our brave fighting men and women are prepared to put life and limb on the line not only for our security but for the security of our whole alliance. It is time that all our allies showed equal financial, military and political commitment."
Dr Fox said it exposed as "fantasy" the idea of replacing Nato with an EU army.
"We need to be the champions of the transatlantic bond that ties us to the United States - our strongest, most like-minded and most dependable ally."
In October Kim Howells, the Foreign Office minister with responsibility for Afghanistan, stressed troops were fighting a "fierce battle" and were in the country for "a long haul".
He said that while British commanders felt they had all of the equipment they needed, they would like more support from some other Nato countries which were not "punching their weight".
Prime Minister Tony Blair is expected to push for a greater contribution to Nato operations in Afghanistan from the UK's allies at the Riga summit, which begins on Tuesday.