A police officer is in a critical condition after losing consciousness during a selection trial for the Special Duties Unit (SDU), also known as the "Flying Tigers" Squad.

It is believed the 31-year-old senior inspector is currently an instructor at the Police Tactical Unit who joined the force about nine years ago.

The officer fell ill while going through an advanced fitness test in waters off Lamma Island at around noon on Thursday.

He was rushed to Princess Margaret Hospital, where he received brain surgery.

Police have suspended the SDU selection process, pending a review to avoid a repeat of the incident.

The SDU is a police tactical unit of the force tasked with countering terrorist attacks, conducting hostage rescue, underwater search and recovery missions, as well as tackling serious crimes involving firearms.