The World Health Organisation’s coronavirus special envoy has warned the Government against releasing lockdown measures until a comprehensive testing, tracking and isolating system is in place.

Dr David Nabarro said rather than giving specific dates for individual measures to be lifted, the Government should say they will release the lockdown “when we know that we’ve got the defence mechanisms in place”.

Dr Nabarro added that the test, track and isolate strategy has “been shown in other countries to be the key to living with the virus”.

He also stressed that a test, tracking and isolating defence system must be in place “right across our societies to keep the ‘R’ number low”.

It comes as teaching unions have called on the Government to “step back” from their plans to begin the phased reopening of schools from June 1, starting with pupils in Reception, Year 1 and Year 6.

Asked how he feels about measures including schools returning from June 1, Dr Nabarro told BBC Radio 4’s Today Programme: “Other countries are saying very clearly we want to make sure that the capacity to test, trace and isolate is present throughout our country before we really start lifting the restrictions on movement.

My request to all countries is that rather than giving precise dates, they say we’ll release the lockdowns when we know that we’ve got the defence mechanisms in place

“After all, what is lockdown? It is freezing the virus in place and it is giving us a time to get ready so that when we release the lockdown we are better protected.

“And I suppose my request to all countries is that rather than giving precise dates, they say we’ll release the lockdowns when we know that we’ve got the defence mechanisms in place, when we know that our people understand the severity of this virus and what we’ve got to do, and when we’ve made sure that in the settings where the virus is most easily transmitted, we’ve put in place the best possible protection to keep those at risk most safe.”

He added that people with cardiovascular diseases and diabetes should keep their distance from others, despite not being advised to shield by the Government.

“I’m very clear that people with diabetes, people with cardiovascular disease are at risk,” he said.

“And it is not so much that I want them to be shielded, it is just that I want them to be given the necessary advice and encouragement to be able to keep their distance from other people.

“And wherever possible, if they are forced to be close together, that we actually do everything possible to avoid them being exposed to infection. We look after them, that is the key point.”