Alan Marshall is selling 20 Humber cars - including a Humber Snipe which took King Edward VIII and Mrs Wallis Simpson round London between 1932 and 1935, prior to his abdication.
The model afforded maximum privacy, with only a tiny window at the back, leaving the couple sitting in the dark, safe from prying eyes.
On its bootlid is a remarkable picnic hamper - complete with porcelain crockery, silver cutlery and a spirit burner.
“I’ve had a lot of Americans coming in over the years on the back of that car”, said Mr Marshall, who was eight when his father acquired a Humber Pullman in 1960, sparking his passion for the cars, which were beloved by Royalty, as well as by every Prime Minister of the day.
Mr Marshall is now selling up, bar his two favourites, but does not want to see a lifetime’s work scattered across the globe.
“I don’t think £250,000 would be out of order when I think a Lamborghini is £300,000 for one car,” he said.
Humbers - created by Thomas Humber (1841-1910) who was educated in Hull - were largely forgotten after going out of production in 1967.