A total of 70,000 people in London are unaware that they have type 2 diabetes and run the risk of developing heart attacks, blindness and amputations, say Diabetes UK.
A Diabetes UK Measure Up Roadshow is to visit five locations in London to provide free diabetes tests, information and advice.
The roadshow tour starts on 19 March and will also visit Redbridge, Wembley, Neasden and Ealing.
Douglas Smallwood, chief executive of Diabetes UK, said: "Diabetes is one of the biggest health challenges facing us today.
"It's shocking that there are thousands of people in London who have diabetes but are going about their daily lives unaware they have a condition that puts them at greater risk of heart disease, blindness and kidney failure and amputations."
He adds that the sooner people find out they have the condition, the sooner they can start managing their diabetes to reduce the risk of life-threatening complications.