David Cameron has put them on standby to head off the ‘fanatical death cult of Islamist extremist violence’.
The elite soldiers will take part in training exercises in which firearms police will re-create the horror witnessed in France last week.
A defence source said: ‘Special forces will be involved in re-enacting the Paris attack because they are the ones who will be deployed immediately should there be a terror threat.
‘They are always on standby but if there is an attack, armed police and the SAS will be ready. Who would you want swooping in through a window rescuing you from a terror attack? You would want special forces – and that is who would come.’
Ministers and officials never confirm the involvement of the SAS in operations.
However, there has been speculation the military helped the police respond to the 2005 attacks in London on July 7 and the failed plot of July 21.
As well as expertise in specialist weapons and hostage-rescue, SAS units can be used for surveillance operations or for ‘spotting’ suspects.
Security officials believe this could be crucial in the immediate aftermath of an atrocity.