Some of the sites like Facebook already have a zero-tolerance policy over comments or messages supporting extremism.
One source said David Cameron, the Prime Minister, wanted the companies to do more to tackle the “scale and reach” of Isil - which is leading an insurgency across Iraq - on the internet which was unlike anything the Government had seen before.
The meeting is being structured around a similar summit last year held to combat the spread of child abuse images online.
Extremist disruption orders or “online Asbos”, which were unveiled by Theresa May, the Home secretary at the Conservative party conference, which will ban certain extremists from the internet could be discussed.
Microsoft and Yahoo as well as internet service providers like BT, Talk Talk, Sky and Virgin are also attending the meeting at lunchtime.
One source at the meeting expressed concern that Number 10 wanted to interfere with a successful flagging system. One Government source said the meeting has been called to see what further the internet companies could do to tackle extremism on the internet.
One internet expert said the meeting was likely to look at the balance between freedom of speech and taking down extremist content.
YouTube, for example, already has a “flagging” system which allow the Home Office to ask it to take down extremist content.
In August Mr Cameron said that Government had helped to take down "28,000 pieces of terrorist-related material from the web, including 46 Isil-related videos".
Some of the sites like Facebook already have a zero-tolerance policy over comments or messages supporting extremism.