Supporters say that advances in technology have made the traditional five-day week outdated and claim that new ways of working should allow staff more time off to spend on hobbies and with loved ones.
But critics have claimed that a four-day week would be dangerous and could hurt the economy.
Union leaders have already backed the case for a four-day working week.
TUC boss Frances O’Grady said that bosses and shareholders “must not be allowed to hoover up all the gains from new tech”.
But even the unions have suggested that the change in working patterns could take until 2100 to achieve.
A Labour insider said that the party was “considering” the idea and that it could come forward with more details this autumn.
Last year the Green Party was mocked when it backed a four-day week.
Research from the International Monetary Fund suggests that the 35-hour week in France had cut employment and not made workers happier.
● Moderate Labour MP Chris Leslie has hit back after his local party voted that they had no confidence in him.
He said: “If the party pushes away people like me on the centre Left for voicing different opinions to the leadership, then that says more about the direction of Labour than it does about me.”