Lawyers Irwin Mitchell are representing more than 80 people who were injured and lost loved ones in the attack in June 2015.
30 British Tui customers were killed during the attack at the Imperial Marhaba Hotel.
Irwin Mitchell represents the families of 22 people who were killed as well as more than 50 people who suffered injuries including gunshot wounds and being hit by shrapnel from explosions.
The law firm said other clients who survived the attack had sustained wounds such as ruptured tendons and leg and feet injuries fleeing gunman Seifeddine Rezgui “while many are still suffering from psychological injuries” after witnessing the incident with some having comforted family members fatally wounded.
Kylie Hutchison, a specialist international personal injury lawyer at Irwin Mitchell representing the families affected, said: “On behalf of our clients who lost members of their families and those who suffered injuries in this terrible incident, we have now served formal civil proceedings on Tui claiming damages.
"The damages claimed will help compensate them for their suffering, their financial losses and help survivors meet the costs of specialist treatments and therapies to aid their recoveries.
“The level of terrorist threat in Tunisia had been escalating for some time prior to June 2015. This included a failed suicide bomb attempt outside a beach Hotel in Sousse in October 2013 and an attack at the Bardo museum in Tunis in March 2015 in which 22 people were killed.
“Despite this Tui, the tour operator who organised the holidays and was responsible for our clients’ safety, did not audit the adequacy of security at the hotel or take appropriate precautions to keep our clients safe from an attack. Nor did they inform our clients of the level of threat of terrorism which many of the holidaymakers say would have changed their mind about holidaying in Tunisia at the time.”
Irwin Mitchell said it believed the case would centre on security at the Imperial Marhaba; what was known about previous attacks in Tunisia and what it claimed to be the "lack of information presented to customers both at the time of booking and when the situation may have changed regarding travel advice".
In a statement Tui said: "We remain truly saddened by what happened on that fateful day in Sousse in June 2015 when 30 of our customers lost their lives in a terrorist attack which started on a public beach.
"Our thoughts remain with all of those who were affected by the horrific incident. As this is now subject to legal proceedings it would be inappropriate to comment further at this stage except to say we will fully cooperate with the judicial process."