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A motorist was robbed of £600 and a diamond engagement ring as he sat in his car outside the QEQM hospital.
Police say the 32-year-old from Margate was approached by his assailant at Ramsgate Road, Margate.
The man asked for directions, then leant in through the car window, grabbed the victim and demanded money.
The offender is said to be black, aged between 30 and 40 with a shaven head.
He spoke with a possible London accent and wore a black coat. He headed off towards Margate.
A police spokesman said: "Inquiries are continuing and we are studying CCTV film from the area.
"Officers also carried out investigations at the hospital and at nearby houses."
The robbery is said to have happened between 9.40am and 9.50am on Thursday, July 3.
Anyone with information should contact detectives on 01843 231055.