The Culture Secretary Maria Miller has summoned them to a meeting on 17 June where they will be expected to come up with plans to do more to stop access to material such as child abuse images or material designed to promote terrorism.

In her letter to the companies, the culture secretary says there is widespread public concern:

"Whether these concerns focus on access to illegal pornographic content, the proliferation of extremist material which might incite racial or religious hatred, or the ongoing battle against online copyright theft, a common question emerges: what more can be done to prevent offensive online content potentially causing harm?"

In a briefing to journalists, the language is even blunter.

"Woolwich is the latest catalyst," an aide to Ms Miller says. "Enough is enough - concentrated effort is now needed by the whole industry."

What this effort is expected to achieve in terms of concrete action by the companies is a lot less clear. They may point out that a whole lot of different concerns, from child safety to copyright infringement, are being lumped together.

In the case of child abuse images (not "child porn" as some government officials still erroneously describe it) the law is already clear - they are illegal. The Internet Watch Foundation maintains a blacklist of such images, updated twice a day, and its members agree to block them - though there will be pressure on the industry to do more to fund the IWF's work and be more proactive in hunting down offending sites.

But when it comes to other material that may be considered harmful but is not yet illegal, the internet industry may struggle to respond to Ms Miller's demands for action. The companies will ask who is to decide exactly what is harmful and should be prevented from "potentially causing harm", and they will not be keen to be cast in the role of internet censor.

The Department of Culture Media and Sport (DCMS) is suggesting various ideas to be discussed at the summit. They include "a set of industry-wide protocols around how they respond to concerns around content when they arise, a look at the extent to which filters can be used, looking further at the public wi-fi proposals beyond the pornography focus". It is also pointing to the code of practice agreed by the mobile broadband companies which restricts access to certain content on mobile phones and asking why the likes of Google, Yahoo and Microsoft can't come up with something similar.

The government is stressing that it isn't going into the summit with a list of demands, but, in the words of one official: "We're saying this is a serious issue, there's a lot of concern and we want you all to think about what you can do."

The companies invited to the summit are keeping their heads down for now. But I imagine conference rooms at Google, BT and Microsoft will be booked out for the next fortnight as worried executives try to come up with simple solutions to make the internet a safer place.