Hans Kluge said the jabs were a vital way of keeping the vulnerable safe.

His comments seemed to be a contrast to other senior officials at the organisation who have criticised countries including the US and Israel for rolling out a booster programme when so many people are still unvaccinated,

He said: “A third dose of vaccine is not a luxury booster [that is] taken away from someone who is still waiting for a first jab. It’s basically a way to keep the most vulnerable safe.”

The WHO Europe boss said 33 out of 54 countries in WHO’s European region had seen rises in cases of at least 10% in the last fortnight.

He added: “More and more studies show that a third dose keeps vulnerable people safe, and this is done by more and more countries in our region.”

He said the rise in cases combined with a slowdown in vaccine uptake was “deeply worrying”.

Several European countries have been hit with anti-vaccination protests in recent weeks.

Thousands protested in the streets of Berlin on Sunday for a second day to hit back at vaccines and restrictions.

Also, Mr Kluge said several countries have been experiencing an increased pressure on hospitals amid the pandemic.

The UK government has said it will start rolling out third vaccines from September.

The most clinically vulnerable will be the first people to be offered the third jab.

In the successful rollout, 42.6 million people have received both doses.

Parents should make sure their children are tested regularly for coronavirus, Education Secretary Gavin Williamson said as he warned youngsters not to get “carried away” when schools return.

The government has been trying to persuade parents, secondary school pupils and college students to take part in voluntary asymptomatic Covid-19 testing.

Concerns have been mounting about the return to classes in England in September and some fear it could drive a new wave of infections.