US commitment to Nato risks being undermined because some European nations are unwilling to deploy more troops in Afghanistan, MPs have warned.
British troops are playing a major role in the fighting in Afghanistan
The Commons Defence Committee said there was a lack of political will among European governments.
In a report on the alliance's future, it said failure in Afghanistan would deal a severe blow to allied unity.
The committee urged Nato leaders to send more troops and with less caveats when they meet in Bucharest next month.
The MPs say they are also worried by what they see as the uncertainty over what Nato is really for these days, which is undermining public support for the alliance.
BBC News defence correspondent Rob Watson said the report was particularly critical of the failure of Nato's European members to spend more on defence.
The committee argued that without more money and more political will there was a danger the US would no longer take the alliance seriously.
Problems were made worse by relations between Nato and the European Union which were "plagued by mistrust and unhealthy competition".
The report said: "Without US support, Nato has no future. But US support depends on Nato becoming more capable, deployable and flexible, and on the European allies contributing more."
The committee urged other countries to share more of the burden in Afghanistan, with the UK, US, Canada and the Netherlands doing most of the fighting there.
It said: "While failure in Afghanistan would not herald the demise of Nato, it would deal a severe blow to allied unity and prompt the United States to question the alliance's continuing utility."
Only six of Europe's 24 members have achieved a commitment to spend at least 2% of GDP on defence.
The committee said: "If the European members of the alliance want to be taken seriously, if they want the United States to remain engaged in, and committed to, Nato, and if they want greater influence in the overall direction of alliance policy, they must commit the necessary resources and improve their capabilities.
"We are concerned that an alliance with such large and growing discrepancies in defence spending will not be sustainable in the long term."