But critics have expressed doubts about the plans, with one describing the idea as a “soundbite not a solution”.
Interviewed on the BBC’s Sunday Politics current affairs show, Mr McDonnell said workers should benefit from increased automation and artificial intelligence in the workplace.
He said: “That might mean reducing hours of work. We are a long hours economy – we work the longest hours in Europe and yet we are less productive.
“The Germans and French produce in four days what we produce in five and yet we work the longest hours.
“I think we are going to be exploring a whole range of issues around automation in particular.
“We will look at the working week because I think people are working too long.
“We will see how it goes.
“But I tell you, in my constituency, people work at Heathrow Airport. They work long hours.
“I went to a school play where when of the children in the play said to their parents, ‘I never see you because you pass in the night on different shifts.’
“It breaks down family life.”
His comments echoed those of TUC general secretary Frances O’Grady, who said last month: “I believe that in this century we can win a four-day working week.”
The idea failed to impress Mike Cherry, chairman of the Federation of Small Businesses (FSB) who said: “Of course businesses should look at using technology and AI to improve productivity in our economy, as the UK lags behind our competitors on the global stage.
“But it’s hard to see how this can be achieved by simply reducing working hours.’
A Confederation of British Industry spokesman said: “At a time when flexible working is becoming more essential than ever, rigid approaches feel like a step in the wrong direction.
“Businesses are clear that Labour should work with them to avoid policies that work as a soundbite but not a solution.”
Conservative Party chairman Brandon Lewis said: “Labour again looking to do untold damage to our economy, which would hurt our productivity, reduce living standards and create higher unemployment.”
The Conservative Party introduced a three-day week in 1974 in a bid to conserve electricity in the face of industrial action by coal miners.
Commercial users of electricity were limited to three specified days’ consumption of electricity a week and banned from working longer hours on those days.
Essential services such as hospitals and supermarkets were exempt, while television companies were required to cease broadcasting at 10.30pm.
The strict measures lasted from January 1 to March 7.
A Labour spokesman said Mr McDonnell’s four-day week idea was not official party policy.