A teenager was taken to hospital with serious head injuries after a moped crash in Cheadle Hulme.
The 18-year-old man was riding the moped when it crashed into a stationary vehicle on Thorn Grove on Saturday afternoon.
The biker had been spotted by police in the Smithy Green area shortly before the crash.
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The officers were investigating a separate robbery at the time.
North West Ambulance Service and the air ambulance attended the crash on Thorn Grove.
The young man was taken by road ambulance to hospital where he is receiving treatment for head injuries.
Greater Manchester Police have referred the crash to the Independent Office for Police Conduct (IOPC).
Superintendent Mark Kenny, of GMP, said: "Investigations are under way by our Serious Collision Investigation Unit to establish the circumstances leading up to the crash. We have also voluntarily referred the matter to the IOPC.
"We also want to speak to anyone who may have witnessed either the robbery or the crash."
Anyone with information should call police on 0161 856 9625. quoting incident 1601 of 17/072021.
Information can also be reported online or by using the Live Chat function at www.gmp.police.uk.
If you can't report online, call 101 or Crimestoppers anonymously on 0800 555 111.
