An earlier lockdown would "likely have saved" the lives of London bus drivers who died with coronavirus, according to a study.
University College London (UCL) was asked by Transport for London (TfL) to investigate the high death rate of London bus workers.
Thirty-four London bus workers died with Covid-19, including 29 drivers - 3.5 times the rate of other roles.
An earlier lockdown "would likely have saved more lives," the report found.
Professor Sir Michael Marmot, who led the study, said: "Our review explicitly suggests that lockdown was the main factor that saved bus drivers' lives.
"Because London was an early centre of the pandemic, it is likely that the increased risk among London bus drivers is associated with exposure."
The report was commissioned by TfL amid major concerns over the deaths of bus drivers in the capital.
Strict guidelines limiting people's movements to tackle the spread of coronavirus were introduced on 23 March.
Since 15 June it has been compulsory to wear a face covering while travelling on public transport in England.
London's coronavirus toll
The mortality rate in male London bus drivers aged 20-64 was 3.5 times higher than men of the same age in all occupations in England and Wales.
UCL's Institute of Health Equity found drivers were at risk because of their job. Many also had underlying health conditions such as high blood pressure.
In total 44 London transport workers died after catching coronavirus.
Figures obtained by Caroline Russell, London Assembly Member for the Green Party, show that 31 out of the 44 deaths were among black or Asian members of staff.
Ms Russell said: "Bus driver deaths have been a shocking reminder of the toll of coronavirus on London.
"The report rightly highlights the need for faster action and a more uniform approach to bus driver wellbeing among bus companies, which the Mayor should lead."
The report found bus workers were more likely to live in the boroughs hardest hit by the virus, and to be from black and ethnic minority groups, which "are more at risk of becoming severely ill and dying from COVID-19".
'Deeply personal' - Sadiq Khan
The second part of the review will explore the differences in infection and death rates between frontline transport workers and London's population in general.
Mayor of London, Sadiq Khan, said: "As the son of a bus driver, this is deeply personal to me, and the transport workers who have lost their lives are constantly in my thoughts.
"I am determined to do all I can to protect staff by putting into practice the recommendations made."
Lilli Matson, TfL's head of health and environment, said: "We have been devastated by the tragic deaths of our colleagues during this pandemic.
"The recommendations in this report provide a roadmap for action to further protect bus drivers.
"It is clear from this piece of work and others that there are certain characteristics that make people more vulnerable to coronavirus."