Matt Ratana shooting: Suspect formally arrested over Kiwi London cop's death

British police have arrested a man in relation to the death of Kiwi police officer Matiu (Matt) Ratana.

Ratana, 54, was shot at Croydon Custody Centre in September. The suspect, 23-year-old Louis De Zoysa, was initially arrested for possessing ammunition and Class B drugs, but has been in hospital since with a critical gunshot wound.

He was formally arrested for the death of Ratana on Saturday (NZ time), BBC News reports, after his condition stabilised.

"It's an important milestone and hopefully it brings some tiny comfort to Matt's partner Su, Matt's wider family and everyone who has been affected," said Met Police commissioner Cressida Dick.

"The investigation is moving on into a dreadful, dreadful homicide."

Ratana was laid to rest earlier this month.

In addition being an officer in the London Metropolitan Police for nearly 30 years, Ratana was the head coach of East Grinstead Rugby Club in West Sussex.

Before moving to the UK, he studied town planning at Massey University in Palmerston North. He also attended high school there.