A man has been arrested in Italy after allegedly kidnapping a young British woman he planned to auction on an online slavery market.

The 20-year-old model was under the impression she was travelling to Milan for a photoshoot, but instead was drugged, hustled away in a suitcase and handcuffed in a house in northern Italy before being freed, Milan police said on Saturday.

Police released a photograph of the suspect who they identified as Lukasz Pawel Herba, a Polish citizen based in the UK.

Herba advertised the "sale" of the woman on the dark web, while at the same time demanding ransom from the woman's agent of $300,000 (about 260,000 euros), police said.

Authorities said as far as they know, no ransom was paid.

Herba was jailed for suspected kidnapping for extortion purposes.

Police official Lorenzo Bucossi told reporters the victim was abducted on July 11 and freed on July 17.

'Attacked, drugged, handcuffed'

A statement from Milan police detailed the woman's ordeal.

"Attacked, drugged, handcuffed and closed inside a suitcase, that's how a 20-year-old English model was kidnapped on July 11 in Milan to be sold to the best offer on pornography sites," the statement said.

Herba was arrested on July 18, the day after he allegedly released the woman and dropped her off at the British consulate in Milan.

The woman had arrived in Milan on July 10 and was supposed to do the photography session the next day, according to officials.

A photographer had booked the session through the model's agent, but as soon as she stepped inside the Milan apartment for the appointment, she was attacked by two men, according to the police account.

"The kidnappers loaded the suitcase with the girl [inside] into a car trunk" and drove to a rural home in a hamlet outside Turin, the statement said.

In the house, "the model was kept handcuffed to a wooden dresser in a bedroom" until she was released on July 17, police said.

Investigations are being conducted Poland, Britain and Italy.

Investigators are trying to determine if the suspect had accomplices and was motivated by a ransom, or was attempting to defraud someone who may have been willing to pay money online for the woman, police said.

They did not identify the model's agent.

Milan daily Corriere della Sera said the kidnapper let his victim go because he discovered she had a child and considered her unsuitable for the sex trade but the police official, Bucossi, told reporters it was unclear why the woman was released.

Around 46 million people worldwide lived in some form of slavery in 2016, according to the Global Slavery Index.