Nato must show that it still has the political will to fight back in the event of a Russian attack on any member of the alliance, Foreign Secretary Philip Hammond has warned.
As alliance leaders - including US President Barack Obama, German Chancellor Angela Merkel, and President Francois Hollande of France - prepared to head for Wales for the Nato summit tomorrow, Mr Hammond said they needed to make clear there was a "red line" around the 28 full member states which could not be crossed.
In a speech to Nato parliamentarians in London, he warned any perception of a weakening in their commitment to collective self-defence could see the alliance, which has been the bedrock of Western security for more than six decades, rapidly unravel.
His comments came amid growing nervousness among former Eastern bloc member states – such as Estonia, Latvia and Lithuania – that they could be the next target of Russian aggression following Moscow's incursions into eastern Ukraine.
Leaders of 28 Nato member states and 30 countries that are partners will be attending the summit which is the first of its kind to be held in Britain for 30 years.
Unlike Ukraine – which only has a partnership relationship with Nato – the three Baltic nations are all full members and, under Article V of the North Atlantic Treaty, the whole alliance would be obliged to come to their defence if any of them were attacked.
Nato leaders are expected to agree the formation of a new, high-readiness "spearhead" force which could see thousands of troops deployed within a matter of days – drawn up partly as a response to Russia's actions in Ukraine.
In his speech, Mr Hammond said it was now clear that efforts to draw Russia into a rules-based system of international relations had failed and that under President Vladimir Putin, Moscow had chosen "the role of pariah".
He said it was essential that Nato demonstrated it had both the will and the capability to respond "rapidly and flexibly" to any threat to its security.
"For Nato to maintain its credibility in the future, we have to demonstrate that we maintain the political will to act to defend ourselves and our interests," he said. "We have to be clear that we have not lost the appetite to intervene when our interests or our obligations require us to do so. If we lack, or are perceived to lack, the political will to respond – and to do so quickly – the credibility of that commitment to collective defence will be undermined, and the very fabric of this, most successful of alliances, will unravel.
"So we have to be clear with Russia, in particular, that while we support a political resolution to the situation in eastern Ukraine, there is a red line around Nato member states themselves that cannot be crossed."
With only four member states – the UK, the United States, Estonia and Greece – meeting the Nato commitment to spend at least 2 per cent of the GDP on defence – Mr Hammond issued an unusually blunt warning that the rest of the alliance must take up their share of the burden.
"That, frankly, is nowhere near good enough. European Nato allies cannot expect the US to continue to carry the burden of defending Europe if European taxpayers are not willing to invest in their own security," he said. "Bluntly, there is no more important task than protecting our security and all Nato member countries now have to quit prevaricating, and put their money where their mouths are."
While officials expect fresh promises from leaders at the summit to raise spending on their militaries as their economies improve, it is unclear to what extent they will be prepared to commit to a firm time-frame.
Meanwhile Russian military forces have been spotted in both major rebel-held cities in eastern Ukraine, an official has said, prompting Ukraine to declare that it now has to fight the Russian army, not just the separatists.