Almost half take days, weeks or months to notice suspicious activity.
Only a quarter of British companies are confident that they can detect a cyber-attack within minutes of it occurring, according to research from Intel Security.
Just under half said it would take a matter of days for them to notice suspicious behaviour on their networks, and might even take weeks or months.
Raj Samani, EMEA CTO for Intel Security, said: ""You only have an advantage over your attackers when you address the time-to-discovery challenge.
"IT departments are inundated by alerts every day and the job to sift through threat data becomes a huge task."
More than two-thirds of UK organisations investigated at least 10 attacks last year, with the same amount claiming that targeted hacking was a primary concern for the organisation.
The trend is leading the cybersecurity industry to increasingly sell products that take advantage of analytics systems to identify useful data amongst the plethora of information found on customer networks.
"Real-time, intelligence-aware, SIEM technologies minimise time to detection to proactively prevent breaches based on contextualisation of indicators during analysis and automated policy-driven responses," Samani added.