There is “substantial uncertainty” about what the widespread reopening of schools will do to the transmission rate of Covid-19, government scientists have been warned.

A new paper presented to the Scientific Advisory Group for Emergencies (Sage) says that schools’ reopening is known to increase the transmission of influenza, but it is not yet known what will happen with the coronavirus.

The Academy of Medical Sciences’ report, published today, warns that the current available data comes from a period where schools were partially closed because of the coronavirus lockdown.

Study: Research to be launched into the impact of school closures

Background: DfE has ‘low confidence’ pupils won’t spread Covid-19

Guidelines: Teachers to go in without the need for PPE

And it calls for intensive surveillance as schools reopen fully in September to understand and track transmission of the virus.

Coronavirus: The importance of well-ventilated classrooms

It also warns that the transmission of the virus could increase in the winter if people in buildings such as schools do not open windows.

The report says that “poor ventilation and overcrowding” increase the density of the virus particles in a room.

It says that a large proportion of UK buildings, including schools, are reliant on people opening windows and vents to ventilate them.

And it warns that in wet and windy weather people are less likely to open windows, meaning their buildings will be less well ventilated.

The government has said that all schools should reopen in September and has produced guidance on how it expects them to do so.

Earlier this year, the Department for Education published a summary of the scientific advice it received from Sage, revealing that there is a “low degree of confidence” that children transmit the virus less than adults.

And, earlier this month, education secretary Gavin Williamson said “it would be very misleading” to imply that the reopening of schools to certain year groups last month was linked to the spike of coronavirus cases in Leicester.

Another major report published today suggests that schools should consider face masks for teachers and older students during the Covid-19 crisis.

This recommendation - which is at odds with the Department for Education guidance - has been made by the Data Evaluation and Learning for Viral Epidemics (DELVE) group, which advises government scientists