Vladimir Putin's advisers are scared to tell him the truth about the progress of his Ukraine invasion but the extent of the Russian leader's "misjudgements" must be "crystal clear to the regime", Jeremy Fleming has said.
In a rare public address during a visit to Australia, the head of Britain's GCHQ spy agency said Putin had "massively misjudged the situation".
And he warned China not to become "too closely aligned" with the Kremlin.
He said: "It's clear he misjudged the resistance of the Ukrainian people.
"He underestimated the strength of the coalition his actions would galvanise. He under-played the economic consequences of the sanctions regime. He over-estimated the abilities of his military to secure a rapid victory.
"We've seen Russian soldiers - short of weapons and morale - refusing to carry out orders, sabotaging their own equipment and even accidentally shooting down their own aircraft.
"And even though we believe Putin's advisers are afraid to tell him the truth, what's going on and the extent of these misjudgements must be crystal clear to the regime."
He added: "It's become his personal war, with the cost being paid by innocent people in Ukraine and increasingly, by ordinary Russians too."
Speaking at the Australian National University in Canberra, Fleming said western allies were making "deeply secret intelligence" public to get ahead of Putin's information war, while also tackling cyber threats.
On China, he said the country's long-term interests are not well served by an alliance with a leader that "wilfully and illegally" ignores the international "rules of the road".
His intervention comes after Prime Minister Boris Johnson last week directly confronted President Xi Jinping over Beijing's stance on the conflict in Ukraine in what was described as a "frank and candid" discussion.
On Thursday, Foreign Secretary Liz Truss will urge Indian prime minister Narendra Modi to work with other democracies to counter Putin's aggression in Ukraine amid reluctance to publicly condemn the actions of Russia - a long-standing ally dating back to the Cold War.
India, which is heavily reliant on Moscow for arms imports, has abstained in a series of votes at the United Nations on the issue.
The UN refugee agency said four million people have now fled Ukraine since Russia launched its war on February 24.