A profit and revenue warning sent shares in 21st Century Technology tumbling 41.9 per cent.The AIM-listed company, which supplies on-board CCTV systems to Arriva and FirstGroup buses, said that after satisfactory first half, difficult market conditions had persisted and its order pipeline for the second half was now significantly lower than expected.Chairman Jon Holmstrom said: "We have received indications that revenue receivable from one of our largest customers is likely to be 70% lower in the second half of 2013 than the first half and below our previous expectations, whilst a four-year contract with the GoAhead Group is due to expire very shortly." He said that having reassessed the order book and pipeline opportunities, the company anticipated its financial performance for the full year to be "significantly below its previously expectations and of the previous year."The company now expects second half sales to be between £4.0m and £5.0m. The lower-than-expected revenues could push 21st Century to a loss. The company gave guidance for the full year of between a loss of £0.25m and profits of £0.25m. In May it said it was expecting to make a full year underlying pre-tax profit of around £1.5m, excluding £0.5m of investment costs.First half sales were down to £5.8m from £7.0m and pre-tax profits fell to £0.48m from £0.68m. The company paid its maiden dividend of 0.7p a share in June. Shares were down 41.9% to 6.25p at 13:03 on Thursday. TB