Here is a look at some of the reasons the West is against policing a no-fly zone.
What is a no-fly zone?
A no-fly zone, also known as a no-flight zone, or air exclusion zone, is a territory or area established by a military power over which certain aircraft are not permitted to fly.
This could include jet fighters, drones or helicopters.
There are several articles in the UN Charter that can serve as a basis for establishing a no-fly zone, such as Chapter VII, Articles 39 and 42, in which the Security Council can identify a "threat to the peace, breach of the peace, or act of aggression" and authrorises the use of force to restore the peace.
No fly-zones have been used in the past during the Bosnian War in 1992 so the airspace could be cleared for delivery of humanitarian aid and during the first Libyan Civil War in 2011 to protect civilians from airstrikes and missiles.
It could lead to the World War Three
The prime minister made clear during his trip to Estonia on Tuesday that having British service personnel enforcing a no-fly zone would be likely to mean “shooting down Russian planes”.
He said such a move would be an escalation of the conflict, and effectively mean the UK fighting Russian forces in Ukraine. ITV News Political Editor Robert Peston said that would be an act of war by a Nato country against Russia and at that point we would genuinely be seeing the start of World War Three.
What You Need to Know - Listen now
The Nato alliance, including the UK, possesses nuclear weapons, as does Russia, meaning involvement by its members could lead to nuclear warfare, with devastating consequences.
Russia's President Vladimir Putin raised the alert status of his nuclear arsenal to “special combat readiness”, the move was a clear signal to Nato: Keep Out.
General Sir Adrian Bradshaw, Nato’s former deputy supreme allied commander, said Nato entering the struggle would “amount to 30 countries against Russia”.
“This is the Third World War in anybody’s language. We cannot afford to let that happen,” he told BBC Radio 4’s Today programme.
If a no-fly zone was established, then Nato aircraft would have to attack Russian surface-to-air missile sites, airfields, radar installation and other military infrastructure, which would ultimately lead to World War Three - something Nato and the world is keen to avoid.
What arguments have been made for a no-fly zone?
Ukrainian journalist Daria Kaleniuk made an impassioned plea to Mr Johnson to enforce a no-fly zone over Ukraine during a visit he made to Poland.
Her argument is World War Three is inevitable - there's no escaping that - and she believes it would be over more quickly with less suffering if the West were to intervene now.
Ms Kaleniuk, who is from Kyiv but crossed the border to Poland, told the PM of the "deep fear" Ukrainian women and children feel because "bombs and missiles which are going from the sky".
"You’re coming to Poland, you're not coming to Kyiv," she said, "because you are afraid, because Nato is not willing to defend, because Nato is afraid of World War III, but it has already started."
Speaking after she made her emotional plea to enforce a no-fly zone, Ms Kaleniuk told reporters that without a no-fly-zone, "Ukrainian children, Ukrainian hospitals will be hit with missiles and bombs".
She said that humanitarian aid cannot reach Kharkiv or Kyiv, where there are missile strikes and bombs and in a couple of days there will be "no medicine, no fuel...it's unbelievable human suffering and disaster".
What has Boris Johnson said?
The PM has repeatedly ruled out a no-fly zone and speaking to ITV News after Ms Kaleniuk's plea, he said he was grateful she escalated that question.
"In many ways for many people that is the unspoken question, why not just engage militarily?" Mr Johnson said.
"But that's not something that any NATO member is thinking of doing and there's a reason for that, which is in order to have a no-fly zone above Ukraine in the current circumstances, you would have to take decisions to shoot down Russian jets and that is not something that any, I can assure you, that any western country is contemplating."
Ukrainian planes would not be able to take off either
The restrictions on planes taking to the air would also have to apply to Kyiv’s aircraft, a policy that could help Russian forces on the ground, the UK Defence Secretary said.
Ben Wallace told Sky News that without Ukrainian planes monitoring activity from the sky, the Russian army would be “able to drive around with impunity”.
Much of the damage inflicted so far has come from land-based weapons
There is a question about how much difference a no-fly zone would make to the destruction being wrought by Moscow on the Ukrainian people.
Many of the rockets and missiles being fired on cities such as Kharkiv are being sent from ground weapons, rather than from the air.
However, that could change as Russian troops advance and, as western intelligence suggests, start to besiege urban areas.
ITV News Political Editor Robert Peston on whether the West's stance on no-fly zones is likely to change
Britain is already supplying anti-aircraft weapons to Ukraine
The Defence Secretary confirmed – thought to be for the first time – that the UK has given anti-aircraft weapons to counter Russian jets.
Mr Wallace said the arms were working to frustrate the Russian air force and preventing daytime bombing raids.
He told BBC Radio 4’s Today programme: “The reason they are doing the bombing at night is… because of those weapon systems – the Russians cannot fly much in the day.”
The Ministry of Defence previously confirmed that the UK was supplying anti-tank weapons to the Ukrainians but officials had been reluctant to publicly state what other arms have been supplied.
Ukrainians fighters are not trained to fly Nato warplanes
Away from calls for a no-fly zone, there has been separate pressure for the West to send Kyiv warplanes to use against Moscow.
However, Ukrainian pilots are not trained to fly the majority of Nato planes.
With the defensive alliance not willing to send armed forces into the country, it would be difficult to provide training to allow Ukrainian pilots to learn how to operate such aircraft.
The European Union had, according to Ukrainian officials, been looking to provide as many as 70 fighter planes from the Soviet-era that Kyiv pilots are trained on.
But that plan appeared to have broken down, with Politico reporting that Poland, Bulgaria and Slovakia – from where the MiG-29s were meant to come from – were against the idea, for security and supply reasons.