BT on Monday warned that distributed denial of service (DDoS) attacks are becoming more effective and yet only half of U.K. companies have a response plan in place.

Research published by the incumbent telco on Monday revealed that 41% of organisations globally were hit by a DDoS attack during the past 12 months, and 78% of the victims were targeted more than once.

"DDoS attacks have evolved significantly in the last few years and are now a legitimate business concern," said Mark Hughes, president of BT Security, in a statement.

Indeed, a typical DDoS attack sees a network of hacked computers used by the attacker to bombard servers with sufficient requests that the server crashes. The result is that targeted Websites go offline and networks grind to a halt.

However, to bypass enterprises' automated defences, hackers increasingly opt for multi-vector attacks. These target different layers of the network, use more than one network of hijacked computers, and make use of hacked mobile phones as well as PCs. In addition, attackers have also started to solicit supporters who willingly participate by volunteering their computer to be used in an attack.

59% of the companies polled by BT said that DDoS attacks are becoming more effective at subverting their IT security measures.

"Reputations, revenue and customer confidence are on the line following a DDoS attack, not to mention the upfront time and cost that it takes an organisation to recover following an attack," said Hughes.

58% of the U.K. companies that BT spoke to admitted that DDoS attacks have brought down their systems for more than six hours.

More worryingly, only 49% of U.K. companies have a response plan in place, and only 8% strongly believe they have sufficient resources to defend against a DDoS attack.

DDoS attacks "can have a damaging effect on revenues and send an organisation into full crisis mode," warned Hughes, who said finance, e-commerce companies and retailers are usually hit hardest when their businesses are targeted.

Meanwhile, BT suffered a fault with its broadband network during the course of the weekend that prevented customers from accessing the Internet.

The BBC reported that many users were unable to access