Shadow Chancellor John McDonnell has said that "people in the UK work some of the longest hours in Europe".
In a speech at Labour's annual conference, he pledged that the party would work towards an average full-time working week of 32 hours within the next decade.
Mr McDonnell's team said he'd been referring to a report which used data from the European statistics agency, Eurostat.
According to that source, full-time workers in the UK work an average of 42.5 hours a week, putting them - as Mr McDonnell suggests - above the European average of 41.2.
Austrian workers also worked 42.5 hours while people in Greece and Iceland worked more than 44 hours a week.
But a paper, produced by the Organisation for Economic Co-operation and Development (OECD), has questioned these figures.
Eurostat relies on data submitted by each country.
Some, like the UK, use hours reported by workers themselves through a survey, while others, such as Germany, use much more complex methods, including asking businesses for their data.
The OECD says countries using the "direct" method - simply taking the numbers reported by workers - have a tendency to over-estimate hours worked.
Because of these different ways of gathering the statistics, the Eurostat data is a good source for comparing one country's working hours over time, but not so good for comparing different countries with each other.
The OECD attempted to make adjustments to the figures to give more comparable estimates.
It estimated UK employees worked 38.4 hours compared with, for example, 38.6 in Spain, 40.9 in Portugal, 41.1 in Luxembourg and 42.4 in Poland.
No country, either in the Eurostat or OECD data, worked close to a 32-hour-week average.
Those in the country with the shortest full-time week, the Netherlands, work, on average, 34.9 hours.
Although France is known for having a statutory 35-hour working week, workers actually put in an estimated 40.4 hours on average.
The 35-hour limit is the point at which workers should qualify for overtime.