By Mark Ward BBC News Online technology correspondent

Employers are monitoring net use to find out what workers are doing

Increasing numbers of companies are putting in place filtering systems to stop staff abusing net resources and spending time surfing when they should be working.

The filtering systems are stopping the accidental mailing of company secrets, finding file-swapping systems installed on company networks and uncovering gangs of people sharing pornography.

Companies are adopting the filters in a bid to avoid legal action by staff offended by what other people are using the web to download.

Smut spotting

Using the net to do business has its risks thanks to the proliferation of viruses, spam and other electronic attacks that attempt to wreak havoc inside corporate networks.

Even if you have a small number of people sharing MP3 files they are burning chunks of bandwidth all the time Roger Hockaday, Packeteer

Greater use of the net also brings with it other, more subtle, risks that companies are now starting to tackle.

Paul Rutherford, spokesman for filtering firm Clearswift, said monitoring net use can be a revelation.

"Universally companies are surprised at what they find," he told BBC News Online.

"They are surprised by the level of traffic, by the numbers of e-mail and the amount of time spent on the web," he said.

"They are surprised about the content that people are accessing or sending."

Mr Rutherford said, in his experience, any company with more than 2,000 employees is likely to have one worker running their own business using company assets.

Almost any large company will have small groups of people passing around pornographic images they look for and download from the web.

Some go to great lengths to avoid being found out and often copy images into a document making it hard, but not impossible, to spot with filtering and monitoring systems.

Sound swapping

Roger Hockaday, spokesman for filtering firm Packeteer, said about 80% of businesses are unwitting hosts of peer-to-peer network software.

Mr Hockaday said file swapping programs such as Kazaa thrive on fast networks and many staff use company systems to download music and movies or share them between friends.

Downloading music can hog a company's net connection

"Even if you have a small number of people sharing MP3 files they are burning chunks of bandwidth all the time," he said.

Music and movie files shared in this way can be hard to spot because it travels via the same network port as other net traffic, said Mr Hockaday.

Companies that do nothing about file sharing systems could find themselves sued by entertainment industry groups seeking to end the swapping of pirated material.

Firms such as Clearswift, Packeteer and Mirapoint provide tools and utilities to companies to help them get control of what people are doing with their networks.

Mr Rutherford from Clearswift said firms must work out a policy of what they consider reasonable use of the net and tell workers about it so they know their rights and responsibilities.

He said that some companies have started limiting when people get unfettered use of the net to better help them police what staff do online.

These policies can help stop the accidental release of confidential material and catch people supplying a company's competitors with key information.

It also helps firms comply with laws requiring employers to create a workplace free from offensive material. Companies that take no action could find themselves being sued by workers outraged at what others are doing.

The UK's Information Commissioner has unveiled new rules that put limits on the freedom that companies have to monitor what their employees are doing with e-mail and net access.