LONDON, Feb 11 (Reuters) - BAA Plc, Britain's biggest airports operator, said on Monday a "gradual recovery" in traffic across all its major markets had continued in January -- a sign that people were returning to air travel since the shock of the September 11 U.S. hijacked plane attacks. For the fourth straight month since the attacks devastated demand for air travel worldwide, the monthly rate of year-on-year decline at BAA's seven UK airports had slowed. A total of eight million passengers travelled through the gates of BAA's UK airports in January, a fall of 3.9 percent from the same month last year but a smaller rate of year-on-year decline than October, November and December, said BAA. The North Atlantic market, where Europe's largest airline, British Airways Plc (BAY), gains most of its profit-driving revenue, showed the biggest improvement along with the country's domestic market, said BAA. The combination of the economic downturn and the September 11 attacks on U.S. cities sent the airline industry into a tailspin as demand for air travel plunged, causing two European national carriers, Sabena and Swissair, to collapse under the strain. While the full-service carriers with high overheads and thin margins have struggled in the depressed market, the low-cost airlines such as Ryanair Holdings (RYA) (RYA) and easyJet Plc (EZL) have been thriving on European shorthaul routes. This has boosted traffic at BAA's secondary city and regional airports, with passenger traffic at London's Stansted up 17.2 percent, Glasgow up 4.3 percent and Edinburgh up 10.2 percent in January on the same month a year earlier, it said. "Growth at these airports has continued to be buoyed by strong marketing by low-fare scheduled carriers," said BAA, whose share price closed at 647-1/2 pence ($9) on Friday. Passenger traffic at London's Heathrow airport, the world's busiest international airport, was down 3.9 percent in January year-on-year, against a 6.8 percent drop in December. Traffic at Gatwick fell by 16.4 percent, but it noted that budget carrier easyJet is about to move in on London's second largest airport. "By the end of February, new routes by easyJet will come on line, giving them a total of 22 daily departures from Gatwick, making it the airport's second largest scheduled airline," said BAA. It said cargo tonnage handled at its UK airports fell by 7.8 percent, the 13th consecutive monthly decline, but showed "a reduction in the rate of decline seen over the last few months". The total number of flights dropped 4.4 percent.