Intelligence chiefs fear the jihadis plan to ram the luxury ships using vessels packed with explosives.
Last week we exclusively revealed a unit of specially trained troops have been assigned to protect British tourists in southern Europe and North Africa.
Now an SBS unit from Poole in Dorset is under orders to use attack boats and divers to counter Islamic State at sea.
DANGER: Hundreds of cruise ships travelling through the Med are now seen as potential targets
Intelligence chiefs think the terrorists may attempt to seize liners and murder passengers or hold them hostage.
Hundreds of cruise ships travel through the Med every summer and are now viewed as easy targets.
Suicide boats have been used to attack US Navy ships in the past, holing the vessels and causing mass casualties.
British spooks believe IS fighters are planning to use similar tactics against liners crammed with holidaymakers.
Some cruise ships are already hiring military contractors to provide additional security when travelling through dangerous waters.
One source said: “We know IS terrorists are attempting to bring fear and panic to the Med.
“They want to attack cruise liners with suicide boats. It’s a very effective tactic – the boats are very fast and cruise liners make a big target and the vast majority are unprotected.”
There are fears the extremists will launch sea assaults from their base in the Mediterranean port of Sirte in Libya.
Frogmen used underwater jet packs to infiltrate the port and destroy dozens of Isis vessels.
Commandos have also assassinated terrorist commanders and helped friendly militias to launch attacks against the fundamentalists.
An SBS frogmen team recently carried out a sniping mission against Isis fighters from the sea.
They were equipped with a specially adapted rifle which allows targets to be engaged from the ocean.
Their first shot with the silenced weapon killed a commander.
They dived for a few minutes, then resurfaced and took out another two fighters before submerging again.
The third wave of the SBS attack left several more jihadis dead – and the remaining fighters fled in terror.
The source added: “The SBS will go after the jihadis at sea and in port.
“There will be spy planes up in the air monitoring every vessel which leaves Libya and if it can’t be identified it will be targeted, especially if it’s heading for a cruise ship.”