More than 100 people have submitted claims for compensation after falling ill on a cruise ship.
Lawyers representing passengers who travelled on P&O's Ventura said there had been "repeated outbreaks" of illness onboard between April and June.
About 519 holidaymakers developed norovirus symptoms in May, after the ship departed Southampton for a two-week cruise around the Canary Islands.
P&O Cruises apologised and said it would be "responding to the letter of claim".
Parent company Carnival was accused of downplaying the number of people taken ill during the sailing, claiming fewer than 1% of those onboard had reported symptoms.
But Southampton's port health authority later revealed that, at the height of the outbreak, the number was actually more than 12%.
Problems with the stomach bug had been going on for weeks, according to passengers on previous sailings.
Norovirus is a stomach bug that causes vomiting and diarrhoea, and the germs cannot be killed with alcohol hand gel.
The spread appeared to begin around 20 April, with 92 people taken ill on Ventura's 10-day cruise around Spain and Portugal.
It coincided with an "unseasonal rise" in norovirus cases across the UK, according to the UK Health Security Agency (UKHSA).
Hudgell Solicitors said it was working on behalf of 115 holidaymakers to seek compensation for "illness, loss of enjoyment and losses".
Anne Thomson, a lawyer at the firm, said it was "undeniable" that the illness had been rife on Ventura.
“Given there were repeated outbreaks onboard over consecutive months, we hope Carnival comes back to us pretty quickly," she said.
Hundreds of holidays had been ruined, according to Ms Thomson, who added that the legal action should come as "no surprise" to the cruise company.
At the time, P&O said a "wide range of approved protocols" had been put in place, including closing on-board launderettes and preventing passengers from serving themselves at the buffet.
Ventura was also deep-cleaned and unwell passengers were asked to isolate in their rooms.
Ms Thomson said: “Let’s not forget, it’s not just the time people are confined to the cabin and are too ill to leave, but also the impact it has on the rest of their holiday, as it takes some time to recover from feeling so unwell to again fully enjoy the facilities, restaurants, and locations and days out."
Carnival has six weeks to acknowledge the claims, and then a further six months to investigate and respond.
A P&O spokesperson said: "We are sorry some of our guests may have been affected and we will be responding to the letter of claim in due course.”
A norovirus outbreak was also seen on P&O's Aurora around the same time, new figures have revealed.
More than 300 people -16% of passengers - were taken ill during its 19-night cruise to the Mediterranean, which departed Southampton on 26 April.
And just under 12% of passengers reported symptoms on the following cruise - a three-night trip to Belgium on 15 May.