LONDON (Bloomberg) - Delta Air Lines Inc., Continental Airlines Inc. and Northwest Airlines Corp. began flights into London Heathrow airport yesterday as a new "Open Skies" treaty expands trans-Atlantic air travel.

The European Union (EU)-US accord ends a lock on flights between the US and Heathrow for British Airways plc., Virgin Atlantic Airways Ltd., United Air Lines Inc. and American Airlines under a 1977 agreement. The airport is Europe's busiest and attractive to other carriers as the continent's key hub for premium travel.

"The main battleground is the corporate traveller using the business-class cabin," said Chris Tarry, an independent airline analyst in London. "US airlines which are new entrants to Heathrow can now offer their corporate clients a greater range of opportunities to London and via Heathrow."

Delta, Continental and Northwest are the third-, fourth and fifth-largest US airlines, respectively, and their Heathrow arrivals will be joined by a flight from US Airways Group Inc., the No.7 carrier. Today, Air France-KLM Group will fly from the UK to Los Angeles.

The influx of flights means seating capacity between Heathrow and the US for the summer season is up 21 percent from a year earlier, according to Aviation Economics, a London-based consulting company. Total capacity is up 3.1 percent.

The EU and US agreed to Open Skies last year, after four years of negotiations, amid hopes it would encourage competition and reduce ticket prices. It replaces a series of pacts between individual European countries and the US which dictated where and when airlines can fly.

The EU's regulatory arm estimated last March the agreement will increase annual EU-US traffic by 26 million passengers to about 73 million over five years. It will also bring consumer savings of as much as 12 billion euros ($19 billion), the European Commission said.

"If anyone's looking for a price reduction overnight, that's not going to happen," said Patrick Murphy, a Washington consultant and deputy assistant to transportation secretary under Presidents Bill Clinton and George HW Bush. "The fuel cost increase will overwhelm the benefits of more competition."

US airlines have lifted fares or surcharges 10 times since January 1 as jet-fuel prices rose 80 percent in the past year. EU carriers including British Airways and Aer Lingus Group plc. have also increased fees this year.

Delta's first flight into Heathrow departed last night from New York and landed at 9am London time yesterday. Another flight from Atlanta, where Delta is based, arrived at 1.44pm. The service is part of a revenue-sharing partnership with Air France-KLM, Europe's biggest airline, announced in October.

"Being able to add Heathrow to our portfolio was absolutely key for our corporate customers," said Armin Venencie, Delta's regional manager for Europe, Africa and the Middle East, in a telephone interview.

Opportunities to fly from Heathrow are limited by a flight schedule that's already at 99 percent of capacity, forcing carriers to pay top dollar for operating rights. Continental paid $209 million for four pairs of take-off and landing slots at the airport.