A badge has been created for passengers with hidden health conditions who need a seat on public transport.
Some 1,000 of the "Please offer me a seat" badges will be handed out as part of a six-week trial in London from September.
Transport for London (TfL) said other passengers would be encouraged to offer their seat to someone wearing the badge.
The scheme to help people with hidden disabilities and conditions is the first of its kind in Europe.
Badges for pregnant women are already in use across the capital.
London mayor, Sadiq Khan, said: "We hope that these new blue badges can make a real difference to those who find it difficult to get a seat when they need one, particularly those with hidden disabilities.
"Everyone who travels around London knows about the success of the 'Baby on board' badges.
"I want Londoners to embrace our new trial and help these blue badges become as instantly recognisable, giving confidence to those wearing them on public transport across London."
James McNaught, who previously developed a "Cancer on board" badge, is taking part in the TfL trial.
He said: "Getting a seat on transport when you need it can sometimes be really tricky, especially if the reason you need to sit down isn't obvious to others.
"When I was undergoing radiotherapy for throat cancer, it meant I couldn't talk to ask for a seat and the morphine I was taking made me appear drunk.
"It was a real struggle to get people to understand why I needed to sit down. I'm really pleased TfL is doing this trial. A badge and card could help make a real difference to the lives of people undergoing drug treatment or with longer term conditions or disabilities."
London transport commissioner Mike Brown said: "We appreciate that asking for a seat on public transport can sometimes be difficult, particularly for customers who have hidden disabilities or conditions.
"That is why we are launching this trial and, if it is successful, we will work closely with older and disabled people's organisations to develop the final product.
"I hope that Londoners help make the trial a success and offer their seat to someone with one of the badges or cards who may be in need."
Passengers can contact research agency 2CV by emailing email@example.com if they want to take part in the trial.
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