A bizarre practice in the Netherlands that allows driving instructors to offer lessons in exchange for sex with their students has attracted criticism.

Dubbed a "ride for a ride", it is deemed legal for instructors to receive the unusual service so long as both parties are over 18.

The practice came under the spotlight in the country's parliament last month, when Mr Gert-Jan Segars, a member of the socially conservative opposition party ChristianUnion, questioned its legitimacy.

While prostitution is legal in the country - with sex workers treated as self-employed persons who are allowed to advertise their services in the newspapers and online - Mr Segars argued that driving students do not possess the required escort licence.

They would thus be flouting the law as they are not able to declare the sex services for tax purposes.

While acknowledging that "ride for a ride" might be undesirable, two ministers have said that it is only unlawful if it were the students who offered sex to their instructors.

"It's not about offering sexual activities for payment, but offering a driving lesson," wrote Minister for Infrastructure and the Environment Melanie Schultz van Haegen and Minister for Security and Justice Ard van der Steur in a letter sent to parliament on Dec 8.

"It is important that the initiative lies with the driving instructor, and focuses on offering lessons, with the payment provided in sexual acts.

"When a sexual act offered in lieu of financial payment, that is prostitution."

According to CNN, a press officer for the Ministry of Security and Justice said "ride for a ride" is not known to be a "common phenomenon".

The Telegraph, however, reported that online searches for "ride for a ride" have recently increased in popularity.