Anyone going on a Saga holiday or cruise in 2021 must be fully vaccinated against Covid-19, the tour operator has said.
Saga, which specialises in holidays for the over-50s, said it wanted to protect customers' health and safety.
The firm said it would delay restarting its travel packages until May to give customers enough time to get jabs.
People over 50 in the UK have been rushing to book holidays as vaccinations boost confidence.
"The health and safety of our customers has always been our number one priority at Saga, so we have taken the decision to require everyone travelling with us to be fully vaccinated against Covid-19," Saga said in a statement.
"Our customers want the reassurance of the vaccine and to know others travelling with them will be vaccinated too."
The firm's holidays were due to restart in March and its cruises in April after a long hiatus, but they will now both be delayed.
Saga said that meant all trips before May would no longer go ahead as planned, acknowledging it would be "a huge disappointment" to customers.
"We will be contacting all guests affected to discuss their options," it said.
The firm said its vaccination policy added to stronger safety processes already planned for when its holidays resume.
These include requiring cruise passengers to have a Covid-19 test before their trip, as well as a full medical screening.
Capacity on its ships will also be kept to a maximum of 800 people.
There were some severe covid outbreaks on cruise ships early on the pandemic, before coronavirus restrictions were imposed.
British-registered ship the Diamond Princess, owned by the company Carnival, was quarantined for nearly a month in February in the Port of Yokohama in Japan.
More than 700 of its 3,711 passengers and crew were infected, and 14 died.
The UK has embarked on a mass vaccination programme as Covid-19 cases surge.
People in England are being vaccinated at a rate of 140 jabs per minute, NHS England boss Sir Simon Stevens said this week.
Experts believe in future that airlines, concert venues and restaurants could routinely ask customers to prove that they have been vaccinated.
And last week, London plumbing firm Pimlico Plumbers said that all of its staff would be contractually obliged to get the jab.