Lord Robertson, Secretary General of Nato, said yesterday that the organisation's future was threatened unless European members spent more on defence.
Backing a call by the House of Commons Select Committee on Defence for European nations to increase the amount of resources devoted to their armed forces, Lord Robertson said the choice was "modernisation or marginalisation''.
If the gap between the US and European countries continued to grow, Washington would bypass the alliance and form its own ad hoc coalition as it has done over the Afghan war.
"Unless European countries are capable of investing in the capabilities which will allow them to work alongside the United States then the United States will simply take decisions on their own and that cannot be good even for the United States.
"It's a question of modernisation or marginalisation and that cannot be good for either side of the Atlantic or for Euro-Atlantic security.''
The defence committee said lack of progress at Nato's November summit in Prague could mean the "bonds of interoperability which hold together the US and its European allies will be broken".
But it said the US should address European suspicions about its commitment to the alliance.European countries spend around 1.8 per cent of GDP on defence, compared with 3 per cent in the US.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies