David Cameron has raised the prospect of further sanctions against Russia as the West's relations with Vladimir Putin grew increasingly strained following the apparent shooting down of flight MH17 over Ukraine by pro-Moscow separatists.
In a sign of a hardening attitude, David Cameron said Europe and the West must "fundamentally change our approach" unless the Russian president alters course.
The comments came after a day which saw the government adopt a tougher approach to Moscow, with Russia's ambassador summoned to the Foreign Office and Philip Hammond claiming Mr Putin was not doing enough to use his influence over the rebels in Ukraine.
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The US and EU had ratcheted up sanctions against Russia shortly before the aircraft was brought down with the loss of 298 lives, but Cameron indicated that an even tougher regime might be needed.
Writing in the Sunday Times, he said: "We must establish the full facts of what happened. But the growing weight of evidence points to a clear conclusion: that MH17 was blown out of the sky by a surface-to-air missile fired from a rebel-held area.
"If it is the case, then we must be clear what it means: this is a direct result of Russia destabilising a sovereign state, violating its territorial integrity, backing thuggish militias, and training and arming them.
"We must turn this moment of outrage into a moment of action."
That action must go wider than just bringing those responsible to justice, he said, adding: "If president Putin does not change his approach on Ukraine, then Europe and the West must fundamentally change our approach to Russia.
"This is not about military action, plainly. But it is time to make our power, influence and resources count.
"Our economies are strong, and growing in strength. And yet we sometimes behave as if we need Russia more than Russia needs us and the access we provide to European markets, European capital, our knowledge and technological expertise.
"We don't seek a relationship of confrontation with Russia. But we must not shrink from standing up for the principles that govern conduct between independent nations in Europe, and which ultimately keep the peace on our continent."
'If Putin does not change his approach on Ukraine, then Europe and the West must fundamentally change our approach to Russia,' Cameron wrote
Expressing his frustration at some of his fellow European leaders, Cameron said: "For too long, there has been a reluctance on the part of too many European countries to face up to the implications of what is happening in eastern Ukraine.
"Sitting around the European Council table on Wednesday evening, I saw that reluctance at work again."
Cameron demanded immediate access to the crash site, with the crime scene preserved and the remains of victims treated with "proper dignity and respect".
Moscow must also stop supplying and training the rebels, he added.
If Russia did not "use this moment to find a path out of this festering, dangerous crisis" then "we must respond robustly", Cameron said.
Earlier, the prime minister spoke by telephone to Dutch counterpart Mark Rutte, whose countrymen and women made up the majority of the passengers on the doomed Amsterdam to Kuala Lumpur flight.
They agreed that the European Union should "reconsider" its approach to Russia in light of evidence that the plane was shot down by separatists.
New Foreign Secretary Philip Hammond expressed his frustration with the Kremlin after chairing a high-level meeting of officials, including from the intelligence agencies.
Philip Hammond said 'the world's eyes are on Russia'
He said: "We're not getting enough support from the Russians, we're not seeing Russia using their influence effectively enough to get the separatists, who are in control of the site, to allow the access that we need.
"This has brought the whole international community together.
"This is not about Russia and the West, this is about the whole community demanding that the proper access is made available to this site, the victims are properly recovered and evidence is secured.
"The world's eyes will be on Russia to see if she delivers on her obligations in the next couple of hours."
A Downing Street spokeswoman said monitors from the Organisation for Security and Co-operation in Europe had been given only "limited" access to the crash site.
As well as Rutte, Cameron spoke to Australian premier Tony Abbott about the catastrophe and they agreed to increase pressure at the UN Security Council for investigators to access the site.
A No 10 spokeswoman said: "All three leaders are clear that president Putin needs to actively engage with the international community and use his influence on the separatists to ensure they allow access to the crash site.
"The FCO in London has called in the Russian ambassador to make these points."
There have been reports of evidence and bodies being tampered with at the crash site
Ambassador Alexander Yakovenko was urged to make sure the Russian authorities use their influence to secure access to the site for accident investigators and victim identification specialists, in a meeting with Foreign Office political director Sir Simon Gass.
In a statement the Russian Embassy said: "Ambassador Yakovenko and Sir Simon agreed that the tragedy must be investigated in an objective, open and independent manner, under the auspices of the International Civil Aviation Organisation and with the participation of other international bodies and interested countries.
"Russia and the UK believe that all parties to the conflict are to ensure access of international experts to the crash site. The Russian ambassador stressed that it is counter-productive for governments to announce their versions of the disaster, as this amounts to putting pressure on the future investigation.
"They also discussed the general prospects of a political settlement of the crisis in Ukraine.
"Mr Yakovenko pointed to the necessity of stopping the armed operation of the Kiev authorities so as to create conditions for a wide national dialogue. He expressed his regret at the recent decisions by the United States and the European Union regarding sanctions with regard to Russia which can only encourage the Ukrainian authorities to continue violence."
Experts from the Metropolitan Police are due to arrive in Ukraine today to assist in the grim task of recovering, identifying and repatriating the bodies of those killed.
A team of six investigators from the Air Accidents Investigation Branch is already in the country, working with international counterparts on the next steps in the effort to establish what happened to MH17.