John McDonnell has committed Labour to reducing the average working week to 32 hours within a decade.
Giving his keynote speech to Labour's party conference in Brighton he said a shorter working week would be implemented without any loss of pay.
"I can tell you today that the next Labour government will put in place the changes needed to reduce average full-time hours to 32 a week within the next decade," he said.
He also promised to eliminate in-work poverty in the first term of a Labour government.
"Transforming people's lives means ending the modern evil of in-work poverty, Labour has traditionally been committed to full employment.," he said.
"We have always believed that getting a job should mean you are lifted out of poverty."
In his speech McDonnell also confirmed plans for Labour to fund free personal care for elderly people in England.
The pledge to fund free personal care, which would cost an estimated £6 billion a year in 2020/21, would more than double the number of people receiving state-funded support, Labour said.
Mr McDonnell also said a Labour government will stop the roll-out of universal credit.
He added: "We'll bring in a real living wage of at least £10 an hour. We'll end the barbaric roll-out of universal credit, we'll cap rents and we'll build a million new homes that our people need."
- Does a four-day-week actually work?
According to The Intrepid Camera - a firm operating on a four-day-week in Brighton - the shorter working week has virtually no down sides.
Company director Maxim Grew told ITV News that since trialing the scheme in January, productivity and morale has increase, whereas the frequency of staff sick days has gone down.
The scheme was so successful he decided to implement the 36 hour, four-day-week permanently.
He told ITV News Political Correspondent Paul Brand: "Sickness went down a lot - people not taking days off for illness was quite a noticeable difference.
"Productivity was up and we were doing slightly less on payroll but people were still working a similar amount, so, yeah, it was positives all-round really."
Staff member Rocco Biancardi agreed.
He told ITV News the shorter working week "gives you motivation to work harder on the days you are here, rather than just slogging it 'til the weekend."
Naomi Davison, another staff member, has become the envy of her friends for having to work one less day per week.
She added: "I have time to do stuff for myself, I have time to kind of relax, see friends, see family etcetera and by the time I come back to work on a Monday I feed refreshed and revived."
So, for this company, the answer is yes: a four day working week does work however critics say the policy would be much harder to implement for other companies.