The UK should not lift lockdown rules until the test and trace system is ready to cope with huge numbers of people, a World Health Organization director has warned.

Dr. Hans Kluge, the WHO's chief for Europe, said the Government must only take sure-footed steps and not rush into decisions like scrapping the two-meter social distancing rule.

He said test and trace must be 'robust' and ready for 'aggressive' use if the number of new cases starts to soar again.

Dr. Kluge said: 'The keywords here are to do it gradually. Do it carefully.'

His comments come after data last week showed that the NHS's test and trace system was not able to get co-operation from a third of contacts in its first week. 

Britain is gradually returning to something of a normal way of life as restrictions on spending time outdoors, traveling across the country, and meeting up in small groups have all been lifted.

'Non-essential' shops, including clothes stores, will reopen for the first time today, and face coverings over the nose and mouth are now mandatory on public transport.

And Government ministers are coming under pressure to loosen the rule demanding that people stay 2m (6'6") from others who they don't live with. Scientific evidence suggests the majority of infectious droplets do not travel that far, but the Government is sticking to the measure as a precaution.

Speaking to The Guardian, Dr. Kluge said the timing of the end of lockdown is as important as the timing of its start.

He said: 'We know that early lockdowns saved lives and bought some time for the health system to be ready. 

'But I would rather than instead of looking to the past, jump to the future and say that the question of lifting the lockdown is as important as going to the lockdown.

'The keywords here are to do it gradually. Do it carefully.'

His comments came as the first statistics from England's NHS test and trace system were published last week.

The data showed that the contact tracing system had only managed to get co-operation from two thirds (67 percent) of people who had been close to someone confirmed to have Covid-19.  

Baroness Dido Harding, who leads the test and trace scheme, admitted: 'Is it completely perfect? No, of course, it isn't.'

She added: 'We won't have got all of the contacts. Some were unreachable, some didn't want to provide contacts, some said ''well, I've already told my mates I tested positive''.'  

Critics said the fact that thousands of people either refused to follow the self-isolation rules or could not be contacted at all, was cause for concern and showed the system was still not ready to play a major role in the UK's coronavirus battle.

Dr. Kluge told The Guardian: 'Contact tracing is key especially as the UK starts to relax the social and physical distancing measures. There has to be a robust track and trace system in place of operation.

'I would like to reply [to questions about the first results of the system] and say we need an effective tracking system in place, it is one of the measures that we recommend that are in place now.' 

Dr. Kluge added that with summer holidays on the horizon, it was important that Europe did not become complacent and crucial that governments issue the flu vaccine to those who were vulnerable. 

Under the Track and Trace scheme, Britons who have been in close contact with infected patients are being told to self-isolate for 14 days, even if they are asymptomatic.

The scheme will require anyone who develops symptoms associated with the coronavirus to get tested by ordering a test online or by calling 119.

If a patient tests positive they must remain in isolation and will be asked to share phone numbers and email addresses for close contacts.

Those who are listed will then receive a text message or email asking them to self-isolate for 14 days depending on the point at which they came in contact with the infected person.  

The system is designed to break the chain of transmission as quickly as possible in order to squash potential outbreaks and stop them from escalating.  

The WHO chief's comments come as Mr. Sunak confirmed the government had put in place a 'comprehensive review' of the current social distancing guidelines.

Speaking on Sky News yesterday he said: 'The Prime Minister has put in place a comprehensive review of the two-meter rule. That review will involve scientists, economists, and others so that we can look at it in the round.

'I know that of course, it's the difference between three-quarters and maybe a third of pubs opening, for example, so it's important that we look at it.'