Speaking to the BBC, Adm Sir Tony Radakin rejected Ukraine's call for a no-fly zone saying it would not help tactically and might escalate fighting.
He urged the West to have confidence that they were doing the "right thing".
The invasion was not going well, Russia was becoming less powerful and it cannot continue, he said.
On Britons wanting to join the fight, Adm Radakin said that the "sound of gunfire" was not "something you want to rush to", and urged people to support Ukraine in sensible ways from the UK.
Asked whether Foreign Secretary Liz Truss had been right to say she would support any Briton who wanted to fight, he said: "We can all understand that sentiment, and that sentiment needs to be channelled into support for Ukraine."
Senior UK military officers are genuinely worried that some British troops - regulars or reservists - might try to join the battle in Ukraine and, in doing so, risk handing Russia a propaganda victory.
Last week the Chief of Defence People, Lt Gen James Swift, sent out a message to the chain of command stressing that UK military personnel were "not authorised" to travel there.
He said that if there was any suspicion that troops were trying to make their way to Ukraine then it should be reported immediately to the Service Police.
The message warned that if serving British military personnel went to fight in Ukraine then they were putting not only their lives in danger but they also risked giving "the mistaken perception" to Russia that Britain had sent in troops to engage in hostilities.
Adm Sir Tony Radakin has now underlined that message, saying it would be "unlawful and unhelpful".
At present the MoD does not believe there are any examples of full-time British military personnel going absent to fight in Ukraine. But it's harder for them to keep tabs on reservists who often also have another career.
In his interview with the BBC's Sunday Morning show, the defence chief painted a picture of Russian forces suffering from heavy losses and low morale, with kit failings and a massive military convoy stalled outside the capital, Kyiv.
The Kremlin has lost more troops in a week than the UK did in 20 years in Afghanistan, he said, and some "lead elements of Russian forces" have been decimated.
He described stories of soldiers whose morale had been so knocked they had abandoned the convoy destined for Kyiv to camp in the forest.
On Saturday, Ukraine President Volodymyr Zelensky delivered a fiery speech saying the West's reluctance to implement a no-fly zone over Ukraine had given Russia "a green light" to continue bombarding towns and villages.
But Adm Radakin insisted such an intervention would not help.
"The advice that we, as senior military professionals are giving our politicians, is to avoid doing things that are tactically ineffective and definitely to avoid doing things that tactically might lead to miscalculation or escalation."
He said most of the shelling and destruction was coming from artillery, not Russian aircraft, and to police a no-fly zone could mean taking out Russian defence systems and shooting down Russian aircraft - leading to an escalation, he added.
Speaking later on the same programme, UK deputy prime minister Dominic Raab said he understood Ukraine's cri de coeur but insisted the West had been clear all along that it would not engage in direct military confrontation.
That would give succour to Russian President Vladimir Putin's argument that he was in conflict with the West, he said.
Mr Putin said on Saturday that any such move to implement a no-fly zone would be seen "as participation in an armed conflict by that country".
Adm Radakin also played down Mr Putin's nuclear threats.
The West has to maintain calmness and responsibility and not react rashly to the latest "bizarre or ridiculous comment" from Mr Putin, he said.
"We are prepared, we are professional armed forces, we will approach this conflict with that level of professionalism and responsibility that you would expect.
"We will also be incredibly confident in our ability to face down President Putin," he added.
He gave an insight into relations with his counterpart, Gen Valery Gerasimov, head of Russian armed forces, explaining the the Ministry of Defence has a direct line to Moscow's operational headquarters.
It is tested every day, he said, and he has used the line to tell Gen Gerasimov they need to speak. "I'm waiting for him to come back to me," he added.
Adm Radakin was also asked whether the West would know beforehand whether Mr Putin would use nuclear weapons.
He said he did not want to go into detail but there had been a "remarkable" level of intelligence in the months leading up to the invasion.
"There are some more discreet elements in terms of warning signs if this was going to start to chart a path towards nuclear escalation," he said.