Virgin Australia and Jetstar are once again cancelling flights to and from Bali after conditions were declared unsafe for flying due to a volcanic ash cloud from Mount Ruang.

Virgin said in a statement that it made the decision to cancel all of its flights today after advice from its team of meteorologists and the Volcanic Ash Advisory Centre.

The company has also delayed all day-time flights from Australia to Denpasar tomorrow as "conditions in the vicinity of Denpasar airport remain unfavourable".

A travel alert will be updated by 10:00am (AEST) on Thursday with further advice.

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The airline said travellers would be able to change their booking to another date or destination within 30 days free of charge.

Jetstar has also been forced to cancel all flights in and out of Bali this evening. A Jetstar flight from Melbourne was also forced to turn back.

The airline plans to resume services from tomorrow morning but will continue to closely monitor the ash cloud overnight, it said in a statement.

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Jetstar customers with valid bookings to or from Bali up to July 31 have the option of moving to an alternative date of travel within 28 days of their original travel date, travelling to an alternative destination within 28 days of their original travel date, or receiving a refund for their original fares in travel vouchers.

Earlier this month the eruption of Mount Ruang created a choking ash cloud that drifted over the resort island, forcing the closure of the airport for a time over the weekend and the cancellation of numerous flights.

Thousands of travellers were left looking for other flights or accommodation, including many who had travelled to Bali during the school holidays.

Virgin Australia and Jetstar added extra flights to help passengers get home and Qantas aircraft were also used to return those stranded.