Maiden Group, the outdoor advertising firm, has issued a surprise profit warning as the fallout from the US terrorist attacks takes its toll on marketing budgets.

The company released the news alongside flat first half results that reflected the impact of the foot and mouth crisis and delays to the general election.

Maiden said pre-tax profits had risen £100,000 to £5.3m, with turnover rising 8% to £41m.

But the market reacted strongly to the profit warnings, sending Maiden's shares down by 13% to a three-year low of 240p.

"The tragic events of September 11 have had an extraordinary effect on what was already a weakening advertising market, and it is now clear that the group will not achieve last year's record pre-tax profits," the group said.

Last year Maiden recorded pre-tax profits of £13m as advertisers turned to the more attractively priced outdoor medium.

But the group was affected by the postponement of the election, which forced advertisers to put their marketing plans on hold.

"The uncertainty that prevailed regarding the timing of the election had a negative effect on the spend form other advertisers, leading to some campaign postponements and some loss of revenue," the group said.

However, it said the UK outdoor market grew by 3% this year, with Maiden outpacing that growth by an extra 5%.