Russia invaded Ukraine on Thursday after weeks of tension in the region as Russian forces amassed on the Ukrainian border. Its actions have been widely condemned, with Boris Johnson promising harsh sanctions. Ukrainian airspace has been shut, and carriers warned to avoid the area.

Several operators are assessing their programmes, most notably Regent Holidays, which is contacting clients due to travel to Russia, Ukraine, Moldova and Belarus in the coming months.

A Saga spokesperson confirmed the over-50s specialist last week rerouted a Black Sea cruise which had been due to call in the Ukrainian port city of Odessa following a change in Foreign Office advice for the country.

"Guests are in the process of being contacted about the change, and the rest of the cruise remains unaffected," said the spokesperson. "We are now looking at the impact of today’s [Thursday] developments on our Baltic cruises later this year, which were due to call in at St Petersburg.

"We will continue to monitor the situation and if necessary make further alterations to our itineraries whilst still ensuring the best experience for our guests."

Carnival Corporation-owned P&O Cruises and Cunard both tweeted: "In light of the current situation evolving in Ukraine, we will continue to monitor very closely and will amend itineraries as necessary following guidance. The safety and wellbeing of guests and crew is of the highest priority and we will advise guests of any changes."

A Riviera Travel spokesperson said the operator was monitoring the situation and would provide updates in due course, if necessary. Riviera is among a number of firms that offer cruises on Russia’s rivers and waterways.