The growth was partly fuelled by the annual Easter advertising uplift, which took place in the first quarter this year, and anticipation of the General Election in May.
The first-quarter results were far higher than the £186.8m earned in first quarter 2004, but only slightly more than the £212.5m generated in the second quarter of 2004, which included Easter last year.
Alan James, chief executive of the Outdoor Advertising Association, predicted that growth would be closer to 10% in the second quarter.
"With Easter coming early, all media benefited from money being pulled into the first quarter," James said. "It's hard to quantify, but the date of Easter does make a real difference. We do anticipate a flattening of growth in the next quarter."
He said that Easter affected all retail advertising, particularly gardening and DIY campaigns.
However, he said the election was not the advertising bonanza it was billed as, because political parties often used advertising as a plank in their public relations strategy – in other words, they unveil a new poster campaign to the press, but don't actually spend a lot on bookings.
The biggest advertiser categories using outdoor in the first quarter were: entertainment and media, up 9.2% to £35.4m; motors, down 4.0% to £20.5m; and finance, up 13.1% to £18.7m.
Advertising by government, social and political organisations rose 48.4%, while retail advertising rose 55.2%.
James said the underlying growth of outdoor was strong, particularly in the six-sheet market, and that reflected the fact that there was better product and more of it.
In 2004, outdoor reached 9.3% share of the display advertising market and the sector has a self-imposed target of 10%.
James predicted outdoor would reach 9.6% by the end of the year.
Maiden Group has warned its shareholders of "challenging" market conditions in the second quarter at the company's AGM.
Chairman Martin Boase said: "While market conditions have been challenging, we did achieve an overall 4% increase in revenues in April and 5% for the first four months compared to last year.
"Growth has been uneven within our three business areas, with billboards continuing to show a decline while being more than compensated by growth across the retail and rail businesses."
He said forward bookings suggested that revenues would be flat in May, while June would bring "modest growth".
He said being named preferred bidder for the Network Rail roadside billboard contract, worth an estimated £350m in revenue over 10 years, meant Maiden could work on reinvigorating its billboard business.
In March, Maiden reported turnover of £92.1m for the full year ending 31 December, 2004, and an operating profit of £3.2m. This converted to a profit on ordinary activities before taxation of £860,000, but a loss on ordinary activities after taxation of £845,000.