Saga’s Spirt of Discovery departed from Tilbury and will call at Portsmouth, Portland, the Isles of Scilly and Falmouth while P&O’s Britannia set off on a four-night sailing from Southampton.
Britannia will sail a series of three-, four- and seven-night UK coastal cruises until 25 September when it begins its western Mediterranean itineraries.
Paul Ludlow, P&O Cruises president, said the line has been on "countdown to this moment".
"A huge thank you to our guests for their patience as we fully recognise that it’s been a period filled with anticipation and thwarted plans," he added.
