London's buses are no longer accepting cash from today as part of a raft of reforms.
Transport for London (TfL) said dwindling numbers of passengers using money to pay for their journeys prompted the change.
Research revealed that 99% of customers already use Oyster, prepaid tickets, contactless payment cards or concessionary tickets.
The move means passengers who do not have one of these tickets will not be able to travel by bus.
But transport chiefs insist the transition should be "trouble free" because of changes which now allow passengers who have run out of credit on their Oyster card to make one more journey.
Mike Weston, TfL's director of buses, said: "The way our customers pay for goods and services is evolving, so we need to ensure our ticketing evolves too.
"Removing cash from our bus network not only offers customers a quicker and more efficient bus service but it enables us to make savings of £24 million a year which will be re-invested to further improve London's transport network."
A decade ago around 25% of journeys were paid for with cash, but now less than 1% do.
Experts say the move is part of a "seismic shift" in shopping habits.
Dave Hobday, managing director of Worldpay UK, the country's leading payment processing company, said: "We're on the cusp of a seismic shift in shopper behaviour, and predict that everyone from farmers markets to buskers will go cashless.
"Contactless kills queues and means businesses never lose a sale during peak times."