"Officers... this morning arrested the brother of Lukasz Pawel Herba in connection with the kidnapping of Chloe Ayling," the National Crime Agency said in a statement.
"Michal Konrad Herba, aged 36, was apprehended at an address... (near Birmingham) on a European Arrest Warrant issued by the Italian authorities."
Lukasz Herba, a 30-year-old Pole, was arrested in Italy this month on accusations of abduction and illegal imprisonment. He was subsequently charged and is in custody awaiting trial.
Ayling, a 20-year-old British mother, claims she was drugged and kidnapped in July after being lured to Milan on the false promise of attending a fashion shoot.
She was taken to a remote mountain village where she was kept for almost a week before Herba drove her back to Milan and released her near the British consulate.
She claims she was told she had been kidnapped by an organised gang and would be auctioned off online as a sex slave.
Detectives are unclear as to whether Herba was involved in a credible attempt to auction his victim or if it was an elaborately staged threat intended to extort a ransom from the model's agent and family.
In an interview with The Mail on Sunday, Ayling acknowledged that many people might not believe her.
"I was in a crazy situation and I was terrified. It has been so frustrating and hurtful to have people not believe me. I know the truth, my family knows the truth and it will all come out at the trial," she said.
Michal Herba will appear at court in London on Thursday, the National Crime Agency said, adding that it was providing support to the Italian police as part of the investigation.
A report in the BBC claims Herba tried to sell Ayling online for £230,000 (approx $300,000). The National Crime Agency is supporting the investigations of the Italian police, according to the same BBC report.