Katrice Lee vanished from the Naafi shopping centre in Schloss Neuhaus, close to the military base where her father was stationed for the army, on 28 November 1981.
She turned two years old that day and had been shopping with her mother.
Despite a vast search at the time by the Royal Military Police (RMP), German police, British troops and volunteers, the trail went cold.
Her case was reopened in 2012 by the RMP, which announced yesterday it would be carrying out a dig on the River Alme’s banks at Paderborner Str, Paderborn.
It followed a reanalysis of evidence gathered during the original investigation, which pinpointed the river as an area of interest, the force said.
Senior investigating officer Richard O’Leary, of the RMP, said: “On this basis, a team of military personnel and civilian forensic experts will be carrying out a detailed excavation of the site, which is expected to last approximately five weeks. The aim of the search is to find evidence that could finally shed light on what happened to Katrice.”
He renewed an appeal for information about a man seen at the shopping centre placing a child matching the toddler’s description into a green saloon car.
“A green saloon car was also seen on the River Alme Bridge near the Naafi the day after Katrice disappeared. It may or may not be the same car, but we are very keen to identify this car,” he added.
A photo-fit of the man has been reissued as part of the fresh appeal.
Anyone from the Paderborn area who owned such a car at the time is asked to approach police so they can be ruled out of the inquiry.
But the possibility remains that Katrice is still alive – now aged 38 – having been abducted and raised with little memory of her early years, RMP investigators believe.
She was born with a distinctive eye condition in her left eye which would have required two medical operations to correct.
Mr O’Leary said: “An age progression image of what Katrice may look like now, aged 38, has been produced.If anyone feels they know someone who looks like this, or believes they could be Katrice, then we would encourage them to come forward.”