The blue “Please offer me a seat” badges and accompanying cards are aimed at people with hidden health problems and are being rolled out after a successful trial on 1,200 people in September, the BBC reported.
The scheme was created after demand from travellers whose hidden conditions meant they struggled to get a seat on the tube.
Nearly three quarters (72%) of those taking part in the trial said making journeys was easier while wearing the badge and 86% said it made them feel more confident to ask for a seat.
The scheme is reportedly the first in Europe to officially recognise hidden conditions on public transport.
Transport users don't need to fulfil set criteria to qualify for the badge and as such the scheme is based on trust like "Baby on board" badges, TfL said. Sadly, this means it is open to abuse.
London’s Mayor Sadiq Khan said the badges “will make a real difference to passengers who need a seat but just haven't felt confident enough to ask for one."
Indeed, people with hidden health problems took to Twitter to welcome the scheme, with some claiming to have had positive experiences wearing it during the trial, and others saying they wished it had been launched earlier.
"This is a brilliant scheme and I am very glad that it is being introduced by the mayor," James McNaught, who took part in the trial, told the BBC.
He previously made his own "cancer on board" badge to increase his chances of getting a seat when chemotherapy and morphine for his cancer made him appear drunk.
"The anxiousness of needing a seat but being unsure whether you will get one can rob people of the confidence to use public transport. This simple initiative will make a huge difference to the lives of many people."