"We have seen an increase of about 80% since April this year," Graham Smith, government and security product manager at C&W told ComputerWeekly.
Cybercriminals have also stepped up the strength of attacks as more organisations roll out DoS protection systems as part of their security strategy.
Attacks have increased in strength from about one gigabit-per-second (Gbps) to around 3Gbps on average in the past year, said Smith.
"This is beyond the scope of most on-premises enterprise DoS protection systems. Only service providers are able to deal with attacks of this size," he said.
At the extreme, some attacks have been as strong as 8Gbps, but so far these have been limited to online gambling services, the traditional target of DoS attacks, he said.
An increased number of attacks has also meant a widening in the types of organisations targeted.
"Attackers are looking beyond online gambling services to include government organisations, commercial enterprises, banks and social networking sites like Twitter and Facebook," said Smith.
Criminals have also switched from using the http internet protocol for DoS attacks to encrypted protocols.
"This makes DoS attacks more difficult to detect and distinguish from legitimate network traffic," said Smith.
DoS detection and protection systems have had to evolve rapidly in response to changes in attack methods, he said.
Although the scale of attacks has increased, criminals are using a greater number of bots or hijacked computer to send low volumes of server requests to avoid detection.
"This increases attackers' chances of success because DoS detection systems take longer to spot these widely distributed attacks," said Smith.
DoS attacks are traditionally aimed at extorting money from victims, but the motive for many of the recent attacks is not as clear, he said.
"Although service providers can shut down these attacks, we can only speculate about the motive because it is very difficult to identify who is behind them," said Smith.