BT’s argument is that filtering web use without the user’s express permission could be regarded as the interception of data.

But children’s charities say the Act only applies to emails and other personal information, not the sites people are able to visit.

One campaigner present at the meeting said: ‘People were completely astonished at BT’s timidity. But equally the fact that the government seem to be dragging their feet hardly inspires confidence.’

BT also raised the possibility of charging companies which use its public wi-fi service extra for imposing a filter. They said it would be on a cost recovery basis, and that they would not make a profit.

But the campaigner said: ‘Filtering porn on wi-fi should be standard. Nobody should have to pay extra to do the right thing.

‘That’s how O2, Sky and Virgin do it. Yet there was a suggestion that BT were going to make it an optional, paid-for extra.’