Delta Air Lines is proposing to operate twice-daily services from London Heathrow to Boston and Miami from spring next year.
The airline has applied to the US Department of Transport and the European Commission for slots that should be relinquished by the Oneworld Alliance as a provision of the British Airways, Iberia and American Airlines merger.
Delta claimed the proposed expansion would be good news for travellers because the increased competition would dent the dominant position of the other airline alliances.
At present Skyteam, of which Delta is a leading member, holds only 5% of Heathrow, while rival alliances Oneworld and Star Alliance command 47% and 25% respectively.
In a statement this morning Delta predicted its new services would start on March 27 next year if its application were successful. Both routes would be offered in co-operation with Delta's joint venture partners, Air France-KLM and Alitalia.
The new routes could put pressure on Virgin Atlantic, which operates one daily non-stop service to Miami and Boston from Heathrow.
Delta would have the advantage of offering a choice of two flights per day and its connecting network throughout the US.
A spokeswoman from Virgin Atlantic told ABTN: "We have operated daily services to both Boston and Miami for a number of years and so have a strong and established presence on both routes.
"If Delta take up the slot remedies for these routes we will continue to compete as vigorously as ever using our superior product and service to attract customers.
"From Virgin Atlantic's perspective the slot remedies which resulted from the approval of the BA/AA deal are entirely inadequate particularly given that competitive lease the slots thereby effectively paying for the consumer harm."