Officers are appealing for witnesses following a collision between a car and a cyclist in Merton.
Police were called by the London Ambulance Service (LAS) at 07:08hrs on Thursday, 20 December following reports of a collision between a car and a cyclist on Parkside, Wimbledon.
The cyclist, a man in his 20s, was taken to a nearby hospital, where he remains with life-threatening injuries.
The car involved stopped at the scene and the driver is assisting police with their enquiries. He was not arrested.
Police would like to speak to anyone who saw the incident or was in the area at the time.
They are also keen to locate a Garmin device believed to have been used by the cyclist.
Anyone with information is asked to call the Serious Collision Investigation Unit on 020 8543 5157 or Crimestoppers anonymously on 0800 555 111.