Women travelling on Berlin’s metro, buses or trams will pay 21% less than men next Monday in a stunt to boost the visibility of Germany’s gaping gender pay gap.

The city’s public transport operator, BVG, said its “Frauenticket” will be available on 18 March only, to mark Equal Pay Day in Germany. Under the slogan “Mind the pay gap”, it said its cut-price ticket was intended to flag the 21% difference between men and women’s average earnings, one of the biggest gender pay gaps in Europe.

“You have to speak out when people are treated differently for no reason,” the operator wrote. “This is a small gesture of solidarity, though it is nothing in comparison with what women are deprived of in income every year.”

The Frauenticket, a day travel pass for the two central zones of the city, will be available for €5.50, down from the usual €7. It will be valid until 3am the next day. BVG said any men found travelling with the discounted ticket would be treated as fare dodgers.

Germany’s seven-decade-old constitution enshrines equality between men and women but statistics show the country lags behind other European nations when it comes to equal earnings. While Italy and Belgium have an average difference between men’s and women’s income of less than 5%, Germany’s figure means it joins Estonia, the Czech Republic and the UK, at more than 20%.

This means a German woman has to work 442 days to earn as much as a man earns in 365, the BVG said. The difference of 77 days means that women, in effect, end up working for free until 18 March each year.

A protester holds a placard at an International Women’s Day demonstration in Berlin in 2019. Photograph: Felipe Trueba/EPA

The operator said just a fifth of its 14,600-strong staff are women, a percentage it is striving to improve. It is run by Sigrid Evelyn Nikutta, who became its first female CEO in 2010 and later made the firm profitable for the first time in its history.

BVG’s gender-specific ticket, a global first, is one of a number of Equal Pay Day special offers being touted in the capital, with restaurants, an optician and a business coach among those pledging 21% discounts to its female customers next Monday. Earlier this month Berlin held a brand-new public holiday to mark International Women’s Day, the only state nationwide to do so.

Berlin’s fleet of bright-yellow U-Bahn (underground) trains play a prominent role in day-to-day life in the city, with Berliners clocking up some 3.5m public transport trips per day. And news of its women-only ticket caused a stir on social media. On Twitter, praise abounded, with many applauding BVG’s offer, despite the odd jibe about its savvy use of cause-led marketing.

But a few men were irked by the plan, with one Twitter user urging other men to buy the Frauenticket, saying they would have a open-and-shut case to sue for discrimination if challenged by ticket inspectors. Another critic quipped: “Will women have to get off the train five stops earlier because they live an average five years longer than men?”

Maren Jasper-Winter, women’s spokesperson for the pro-business FDP party in the Berlin House of Representatives, said the offer “missed the point”. Quoted in the Berliner Morgenpost newspaper, she argued “It’s as if you’d raise a small child’s pocket money just once by €1.50. This doesn’t help any woman and is completely irrelevant in view of the challenges for equal rights in the world of work.”

But BVG said it had anticipated the backlash. “We knew we would be badmouthed for this,” said Petra Nelken, a BVG spokesperson. “Of course this price gap feels unfair. But that’s the whole point: just for a day we just wanted to make the big pay gap feel tangible in ticket form. This is what women are up against every day.”

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