News broke last week that the mayor of Copenhagen, Frank Jensen had banned all of the city's staff from flying with Ryanair, even when it offers the cheapest tickets.

This is a protest against the airline not entering a Danish collective-bargaining agreement.

Ryanair hit back with an advertisement mocking the decision, featuring a crude photoshopped picture of Mr Jensen's face on a portrait of Marie Antoinette:

Here are the Low Fares #Copenhagen's High Fare Mayor @FrankJensenKBH doesn't want you to book! http://t.co/4qBo0gtRof pic.twitter.com/OOF6dDYrY3

— Ryanair (@Ryanair) May 20, 2015

The mayor's fellow Social Democrat politician Peter Hummelgaard responded with the below tweet, saying “Ryanair has for the past few days attacked Frank Jensen with vulgar ads. We have made an equally vulgar response,” signing off with a smilie face. 

Ryanair har de sidste par dage angrebet @FrankJensenKBH med platte reklamer. Vi har lavet et ligeså plat modsvar :) pic.twitter.com/VJkPpTFNQ3

— Peter Hummelgaard (@PHummelgaard) May 21, 2015

Karina Lorentzen, an MP for the left-wing Socialist People’s Party, also joined in - warning the Irish airline not to "mess with" Danish unions.

Ryan Air -"You want war? We get you war!" Don´t mess with our unions :-) pic.twitter.com/KbtmYuN8ac

— Karina Lorentzen D. (@MF_K_Lorentzen) May 21, 2015

The Mayor also accused the company of "social dumping" - this is defined by the European Commission as "where foreign service providers can undercut local service providers because their labour standards are lower."

Ryanair has said that it believes that Mr Jensen is misinformed about Ryanair’s working conditions - and that his decisions have given preferential treatment to SAS, an airline partly owned by the Danish state.

The company also tweeted this response: 

Here's a closer look at the #Ryanair Well Paid Jobs @FrankJensenKBH #NoThanksFrank pic.twitter.com/IzjsrbpuXk

— Ryanair (@Ryanair) May 21, 2015

And this follow-up tweet anouncing a surge in Danish bookings - again featuring Mr Jensen.

We've had record bookings in #Denmark! Thanks Frank! Book our 69DKK fares before he bans them! http://t.co/iSIehuKWL2 pic.twitter.com/HeKFatQ4c4

— Ryanair (@Ryanair) May 22, 2015

Ryanair spokesman Robin Kiely said:

"Ryanair is celebrating a record week in Denmark with bookings up 45 percent, thanks to all of the free publicity generated by Mayor Frank Jensen. We have also received over 100 applications from Danish pilots wanting to join Ryanair and earn salaries of over 1.1m DKK per annum."

In March the Irish airline's first flight from Copenhagen was grounded for three hours amid a dispute with unions over wages and working conditions.

On Wednesday, the Danish Labour Court is expected to rule over whether the company should operate under Danish or Irish laws in the city.

Last month, Ryanair announced that it would expand its Danish operations to include new direct flights to Budapest, Malaga and Alicante.