The European Commission, European Central Bank and International Monetary Fund, known as the troika, are also requiring other radical labor reforms affecting wages, overtime and work flexibility in return for a $37.6 billion bailout next month, the letter leaked to the British newspaper The Guardian said.
The EC, ECB, IMF and government of Prime Minister Antonis Samaras had no immediate comment on the report, which came as troika inspectors returned to Athens for a final assessment to determine if the debt-laden country gets any further money to help it avoid a default and possible removal from the eurozone.
The letter, sent to Greece's finance and labor ministries last week, orders Athens to extend the workweek into the weekend.
"Measure: increase flexibility of work schedules; increase the number of maximum workdays to six days per week for all sectors," The Guardian quoted the letter as saying.
"Increase flexibility of work schedules; set the minimum daily rest to 11 hours; delink the working hours of employees from the opening hours of the establishment; eliminate restrictions on minimum/maximum time between morning and afternoon shifts; allow the consecutive two-week leave to be taken any time during the year in seasonal sectors," the letter said.
Greece must also establish a permanent "single-rate statutory minimum wage" in a country where 23 percent unemployment, forecast to reach 29 percent next year, "is too high, and policies are needed to prevent it from becoming structural," the letter said.
Non-wage labor costs must also be lowered, the letter said among other labor-reform requirements.
The letter shows how deeply the international creditors are willing to intrude into Greece's system and culture of work widely seen outside the country as dysfunctional, The Guardian said.
The troika demands are also likely to worsen a standoff between the government and Greece's organized labor, the newspaper said.
Greece is struggling to cut $14.5 billion from its budget -- mostly from state spending, pensions and social benefits. The cuts were supposed to have been put in place in June and must be in place to get next month's financing.
Athens blames a deep recession for falling behind.
The troika inspectors have said they will deliver a verdict within weeks that could determine if Greece is allowed to stay in the eurozone.
European officials speculate Greece may have to exit the economic and monetary union, but after the November U.S. elections, The Guardian said.