The average working week in the UK would be cut to 32 hours within 10 years under a Labour government, John McDonnell has announced.
This would reduce it to the equivalent of four days - although it would not necessarily mean a day off as other methods could be used to cut hours.
The shadow chancellor said the cut could be done with "no loss of pay".
The average UK full-time working week is 42.5 hours versus an EU average of 41.2, statistics body Eurostat says.
"We should work to live, not live to work. As society got richer, we could spend fewer hours at work," Mr McDonnell told Labour's annual conference in Brighton.
"But in recent decades progress has stalled, and since the 1980s the link between increasing productivity and expanding free time has been broken. It's time to put that right."
Labour pledged to introduce four new public holidays in their 2017 general election manifesto but did not mention working hours.
In a wide-ranging speech, Mr McDonnell also vowed to:
- End the UK's opt out from the EU working time directive, which caps at 48 the number of hours people can work in an average week
- End the "modern evil" of in-work poverty within Labour's first term in office
- Expand public services free at the point of use including childcare, post-school education, public transport for under-25s and school meals
- Make personal social care free at the point of use, at an estimated cost of £6bn a year
Mr McDonnell said negotiations over working hours would be carried out as part of plans to roll out collective bargaining across different industries.
Collective bargaining is where wage rates and conditions are agreed between employees and trade unions, a practice that used to be commonplace in British industry.
"We'll require working hours to be included in the legally binding sectoral agreements between employers and trade unions," said Mr McDonnell.
"This will allow unions and employers to decide together how best to reduce hours for their sector.
"And we'll set up a Working Time Commission with the power to recommend to government on increasing statutory leave entitlements as quickly as possible without increasing unemployment."
The Working Time Commission would also have the power to increase statutory holiday entitlement, which is currently 28 days.
In his speech, Mr McDonnell also defended the Labour leadership's Brexit stance, which would see Jeremy Corbyn promise to negotiate a new deal with Brussels and then put it to a referendum, with the party to decide whether it backs Remain or Leave at a special one-day conference nearer the time.
He said cancelling Brexit, by revoking Article 50, would send out the wrong message.
"We can't say to people 'Labour wants you to share in the running of your workplace, your community and your environment, but we don't trust you to have the final say over Brexit.'"
The shadow chancellor's proposals for a shorter working week were welcomed by the trade unions.
TUC general secretary Frances O'Grady said: "It's time for working people to share in the benefits of new technology."
And the national co-ordinator for Labour campaign group Momentum, Laura Parker, said its members were "delighted" that a policy they had campaigned for was being adopted.
"This is what a democratic party looks like," she said. "Policy is being written by the movement, with members and the leadership working hand in hand to write the next manifesto and deliver the ambitious, radical policies we need to win the next election."
But CBI director general Carolyn Fairbairn, who represents business, said: "Who would turn down a four day week on the same pay? But without productivity gains it would push many businesses into loss."
In an average week, the total of all hours worked by the entire workforce is 1.05 billion. If you assumed the workforce remained the same, then this policy would see total hours worked cut by around 100 million hours.
In and of itself, it would have a significant effect on the economy. But the opposition argue not just that the policy will not cost the economy, but that individual workers will not get a pay cut. How is such a free lunch, indeed tens of millions of such lunches, possible?
It requires an epic increase in productivity, how much each worker actually produces, something that has eluded the UK economy.
Business organisations fear that this is the cart before the horse, requiring huge capital investment.
The opposition are essentially trying to grab the benefits of a future decade of technological advancements towards workers, rather than business owners.
A report by cross-bench peer Lord Skidelsky - commissioned by Labour and published earlier this month - recommended that people should work fewer hours to earn a living.
The report said capping workers' hours to a four-day week would not be "realistic or even desirable", citing France's introduction of a 35-hour working week in 1998.
"The evidence is that, after a brief impact effect, France's legislation was rendered broadly ineffective by an accumulation of exceptions and loopholes," it said.
Labour said its policy would not be a French-style "cap" on hours because it would rely on sector-wide agreements rather than enforcement action against individual companies.