Nato must rethink its long-term relationship with Russia and take measures to ensure that its members can respond quickly to any threat from Moscow in the wake of its "illegal" actions in Ukraine, David Cameron has said.
Measures should include sustaining a "robust" defensive presence in eastern Europe, adopting a new schedule of military exercises, pre-positioning equipment and supplies in key locations and beefing up Nato's Response Force of swiftly deployable land, air, maritime and special operations troops, said the Prime Minister in a letter to his Nato counterparts and alliance secretary general Anders Fogh Rasmussen.
The letter comes just days after a report from the House of Commons Defence Committee warned that the trans-Atlantic military alliance was not adequately prepared for a potential threat from Russia.
Six weeks ahead of the Nato summit in south Wales, Mr Cameron said he wants to use the gathering to agree a new approach to Russia which will send a message to President Vladimir Putin that Nato and its member-states - which include former Soviet republics Estonia, Latvia and Lithuania - will not be intimidated.
The summit near Newport is the first in the UK since Margaret Thatcher hosted the alliance in 1990, as the Cold War was ending and Nato started to chart a course for a new relationship with Russia.
In his letter, Mr Cameron said that this September's gathering marks another "pivotal" moment in Nato's history.
"In 2014, the world is more unpredictable than ever," said Mr Cameron. "To the East, Russia has ripped up the rulebook with its illegal annexation of Crimea and aggressive destabilisation of Ukraine. To the South, an arc of instability spreads from North Africa and the Sahel, to Syria, Iraq and the wider Middle East.
"So we must use the Summit to agree how Nato should adapt to respond to and deter such threats; and to ensure the continued collective defence of all its members."
He said: "We must... review our long-term relationship with Russia. While Nato has only ever sought to be a partner to Russia, not a threat, it is clear that Russia views Nato as an adversary.
"We must accept that the co-operation of recent years is not currently possible because of Russia's own illegal actions in Nato's neighbourhood and revisit the principles that guide our relationship with Russia."
Six months into the Ukraine crisis, Mr Cameron said Nato must agree on "long-term measures to strengthen our ability to respond quickly to any threat, to reassure those allies who fear for their own country's security and to deter any Russian aggression".
He added: "All Nato allies have already contributed to the alliance's response to this crisis and we should agree how we can sustain a robust presence in Eastern Europe, consistent with the Nato Russia Founding Act, to make clear to Russia that neither Nato nor its members will be intimidated.
"We should agree specific actions including: a new exercise schedule adapted to the new security environment; the necessary infrastructure; pre-positioning of equipment and supplies; and an enhanced Nato Response Force. This should be part of a broader action plan that enables us to respond more quickly to any threat against any member of the Alliance, including when we have little warning."
Mr Cameron will visit Nato's Supreme Headquarters Allied Powers Europe (SHAPE) with Mr Rasmussen on Monday, when they will meet the Supreme Allied Commander Europe, US General Philip Breedlove, to discuss plans to strengthen the alliance's ability to respond swiftly to any threat against any member.
The Prime Minister also proposed a new North Atlantic Armed Forces Charter, modelled on the UK's Military Covenant, to demonstrate a commitment to the welfare, medical care and support of troops and their families.
He said the Newport summit must also discuss how Nato can support the Afghan government following the withdrawal of international troops at the end of this year and how the alliance can address the threat posed by fragile states, including by agreeing defence capacity building missions to parts of the world like Georgia and the Middle East.
Mr Cameron urged other member states to match the UK's record of meeting the Nato target of spending 2% of GDP on defence.
And he said that Nato should use the Wales summit - to which 33 partner countries have been invited - to demonstrate its commitment to working with other states which share its belief in an international rules-based order which promotes freedom, democracy and the rule of law.
"At the London Summit in 1990, leaders agreed that `we need to keep standing together, to extend the long peace we have enjoyed these past four decades'," said Mr Cameron. "That remains just as true today.
"By working together we will be stronger together. The Wales Summit should prove that Nato is a rock-solid alliance with strong partnerships around the world that fosters global peace and stability, creating a secure environment for economies to grow.
"It is an Alliance that reassures our 900 million citizens that together we can protect them from the changing and multiplying threats of an unpredictable world."