Britain's four biggest telcos are under pressure from the Prime Minister to describe their forthcoming network-level internet filters as "default on" by 22 July, The Register understands.

A well-placed industry source told us today that Tory leader David Cameron will make an announcement about the web content controls next Monday during a speech about internet safety.

That address will follow attempts by government officials to chivvy Blighty's ISPs into using the "default on" terminology approved by Number 10.

Yesterday, the BBC published a leaked letter from the Department for Education that nudged internet providers into changing the wording of their website-blocking plans from "Active Choice +" to "default-on".

The department's memo was seen as last-minute horse-trading with ISPs to convince them to adjust the phrasing for their plans to offer customers the option of blocking legal sites that contain porn, violence and other material that's deemed to be inappropriate for kids.

Unsurprisingly, the telcos - which are currently not subjected to regulation on the issue of filtering content - are furious with the government's continued meddling.

Cameron has previously described such filters as a "crude system" for stymieing such content. But he now embraces the idea and says, according to Whitehall mandarins:

The prime minister believes that there is much more that we can all do to improve how we communicate the current position on parental internet controls and that there is a need for a simplified message to reassure parents and the public more generally. Without changing what you will be offering (ie active-choice +), the prime minister would like to be able to refer to your solutions are 'default-on' as people will have to make a choice not to have the filters (by unticking the box). Can you consider how to include this language (or similar) in the screens that begin the set-up process? For example, 'this connection includes family-friendly filters as default [or as standard] - if you do not want to install this protection please un-tick the box' (obviously not intended to be drafting). Would you be able to commit to including 'default-on' or similar language both in the set-up screen and public messaging?

One industry source has already told The Reg that it's completely wrong to use the word "default" for describing how the filtering system will work in practice.

But the letter itself may have a point: BSkyB, for example, has already said that its filtering system (which nobbles DNS lookups and will be offered from the end of this year) will come with the box ticked as standard. Arguably, then, it will be on by default because the customer is required to actively switch it off.

In the end, the big ISPs - BT, TalkTalk, Virgin Media and BSkyB who have now all agreed to block controversial content - may simply have been drawn into a fight over semantics.

The battle between the government and telcos over whether or not material should be filtered at the network level appears to already have been won, however. The alternative, as the PM has previously threatened, would have been regulation.

The Internet Service Providers' Association, which represents the country's largest ISPs, criticised Number 10. It told us:

Industry has put a great deal of time and expertise in offering customers a choice over what content filters to use as part of an active choice system that puts parents in control of what content is permitted in their households. Government should be focused on what is most effective – a combination of education and awareness, mediation and filtering – rather than constantly shifting the goal posts.

The leaked letter also contained some other interesting titbits, including more detail on the age verification tool that telcos have been developing. It would appear that a "closed-loop" system will be used that would notify subscribers of any tweaks made to the filters on their service.

And, according to the education department's demands, Number 10 is keen for all telcos to adopt a "browser intercept" option that is being tested by TalkTalk - which was the first big name ISP to implement network-level filtering on its service back in 2011. That tool will nag every household about the opportunity to configure the web-blocking services offered by BT, BSkyB, Virgin Media or TalkTalk - such as leaving the technology enabled, disabling it or adjusting the list of banned websites.

It's understood that telcos are also disputing the government's demands for the industry to agree to a "collective financial commitment" to fund an "awareness campaign" for parents in the UK. But Whitehall is yet to explain how that cash may be spent.

Unsurprisingly, ISPs are reluctant to cough up money without knowing what exactly it will be used for.

We asked the Prime Minister's office to comment on this story, but it hadn't got back to us at time of writing. ®