Eastbourne Buses Ltd says the fare increase - the first since 2005 - is due to rapidly rising fuel costs.
The flat fare within Eastbourne will be £1.70 for an adult. The current fare is £1.50, so the 20p increase represents a rise of just over 13 per cent.
A child's fare will be increased from 80p to 90p, and other fares will be increased by similar amounts.
This means it costs twice as much to catch the bus in Eastbourne compared to Hastings. Stagecoach, which covers Hastings, charges 85p for a single fare and £1.50 for a return.
Brighton and Hove Buses charges £1.20 for a single within the city of Brighton, or £1.70 between Shoreham to Newhaven.
Eastbourne Buses revealed in the Herald of February 15 that the number 7 and number 4 routes were being dropped in a bid to cut costs. A competitor announced this week that it will take over Route 7.
Managing director Steve Barnett said, "In common with all other transport operators we are facing significant cost increases which are outside of our control.
"Everyone who uses cars will be aware how much the price of fuel has increased recently. The current cost of fuel for our buses represents a 46 per cent increase on what we paid in November 2005 – the last time we revised our fares.
"We have suppressed the need to pass on this cost to customers for well over two years but can no longer do so."
An Eastbourne Buses spokesperson stressed there are several deals available that can reduce costs.
For example, the Day Rover ticket for unlimited travel within Eastbourne will continue to be available from PayPoint outlets for £2.50, (£2.70 on the bus).
The popular Saver 7 will be £10 from PayPoint or £10.50 on the bus. A 28-day ticket for only £36 can be purchased exclusively from PayPoint outlets.

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