Englands largest coronavirus infection survey has given a glimmer of hope that the growth rate of the pandemic is beginning to decrease after rising very fast for most of september, as the rule of six and other social-distancing measures take effect.

The study by imperial college london is based on swab tests on a random sample of 84,000 people between september 18 and 26. the preliminary results suggest that the reproduction number r fell to 1.1 from 1.7 earlier in the month, though the researchers said there was a wide margin of error r, the average number of people infected by someone with the virus, could be anywhere from 0.7 to 1.5.

The findings follow a warning on wednesday by uk prime minister boris johnson that more coronavirus restrictions could be imposed soon, as his chief scientific adviser patrick vallance admitted the virus was not under control.

Steven riley, co-author of the imperial report, compared the trajectory of the epidemic to someone climbing a hill and finding that the gradient was becoming less steep. we dont know if we have reached the summit yet, he said.

Paul elliott, director of imperials react survey, added that the recent fall in the r number should not undermine the governments drive for more compliance with social-distancing measures.

While our latest findings show some early evidence that the growth of new cases may have slowed, suggesting efforts to control the infection are working, the prevalence of infection is the highest that we have recorded to date, he said. one in 200 people walking the streets today is infected.

Prevalence of infection rose everywhere, the report found, but remains highest in north-west england at 0.86 per cent. cases increased substantially in london, rising fivefold from 0.10 per cent to 0.49 per cent.

A few hours after the results of the survey were published, the government announced further restrictions across the north of england affecting more than 2m people across seven boroughs in and around merseyside and tees valley.

Under the rule changes it will become illegal to meet people from a different household indoors in pubs and restaurants in the liverpool city region, warrington, middlesbrough and hartlepool.the measures, expected to come into force at the weekend, matchthose imposed in north-east england last week.

Imperials researchers estimated that 0.55 per cent of the population has the sars-cov-2 virus that causes covid-19, compared with 0.13 per cent in the previous round of testing between august 22 and september 7.

Prof riley said: there is initial evidence of a deceleration in the epidemic but it is only preliminary evidence. the key for me is that prevalence has to come down.

Infections are increasing most rapidly in people aged over 65, who had seven times more positive tests than in the previous round. but young people continued to have the highest rates of infection, with one in 100 of those aged 18 to 24 estimated to have the virus.

Commenting on the findings, the imperial college team said in an unpublished paper: the slowing of the rate of increase in prevalence...follows a reinforced public health messaging campaign by the government for individuals to comply with social-distancing measures, restrict numbers of people visiting private households or meeting outdoors at any one time, and, more recently, closure of pubs and bars by 10pm and enforcement of social distancing rules.

Prof riley said the recent deceleration is evidence that people have changed their behaviour during september but the data were not strong enough to link the fall in r to the rule of six coming into effect on september 14 or any other specific measure.

The regional data suggest that the recent increases are not restricted to local outbreaks, although there was some evidence of local clustering. this implies that reinforcement of public health measures on social mixing and distancing needs to occur at the national level, and not only at the local level where hotspots are detected, the researchers said.