UPDATED | Delta Air Lines will slug passengers booked into its new Airbus A350 Delta One business class suites with a surcharge as high as US$1,000 on a return ticket.

The 'Delta One levy' was highlighted in a memo sent this week from Delta to travel agents in Korea, which will host the A350 flights from Detroit and Atlanta to Seoul's Incheon Airport.

Agents have been advised that when they "book the A350-900 with 'Delta One Suite', you will be charged a surcharge" on top of the base fare.

Delta One passengers booked under discounted I-class and Z-class fares will be charged US$500 "per direction", with a US$250 tariff on C, D and J ticket codes.

Based on current average Delta One fares of US$7,600 for a business class round-trip between Detroit and Seoul flown on the airline's Boeing 747 in early November, the A350 Delta One suites surcharge could see that same fare rocket as high as $8,600 – a jump of some 13%.

Asked to explain the rationale behind the surcharge, a spokesman for the airline said "the Delta One suite is priced appropriately and competitively for the premium experience it will offer our customers."

"Other Delta One fares are available to the same destinations on routes and aircraft not equipped with the Delta One suite."

Despite repeated enquiries from Australian Business Traveller, Delta Air Lines refused to confirm if the new Delta One suites surcharge would be applied to all routes flown by the Airbus A350.

However, this have since proved to be the case for all Delta Airbus A350 flights, which take wing  between Detroit and Tokyo from October 30, to Seoul from November 18 and to Beijing from mid-January 2018. A350 flights between Delta's Atlanta hub and Seoul will launch from March 24, 2018.

While Delta will begin upgrading its entire Boeing 777 fleet with the new business class suites from 2018, Australian Business Traveller was told "It has not (been) decided yet if the surcharge applies to B777 after upgrading."