The summer holidays are fading to a distant memory (it will be half-term before you know it) and the supermarket shelves are overflowing with the first Christmas treats – anyone for Quality Street, Celebrations, or Cadbury’s Roses?

But while Sainsbury’s, Tesco and Asda want us to start planning a mere three months ahead, the cruise lines are trying to get their customers to think much further into the future.

P&O Cruises is promising its biggest-ever discounts and asking for deposits of less than £30 per person to secure a summer 2018 cruise.

Reservations opened on September 19. New customers booking before November 30 can claim a 5% discount and will be required to put only 5% down – all of which means that a seven-night Mediterranean cruise could be theirs for £599 and a deposit of just £28.50.

Past guests get an even better deal – a 10% discount.

The P&O package includes 36 school holiday cruises at Easter, summer and the May and October half-terms. Seven new ports include Capri and Rapallo in Italy, Rovinj in Croatia, Kristiansund in Norway, and a trio of destinations in north-east Canada.

There will be plenty of themed cruises to choose from, including the Food Heroes and Strictly Come Dancing.

P&O Cruises’ senior vice-president Paul Ludlow said: “This is the earliest we have ever launched our summer holiday collection. It’s really great for those who like to book a holiday for next to nothing and then have time to save up for it.”

Not to be outdone, Cunard has also unveiled its 2018 programme, with no fewer than 20 maiden ports of call – on sale from September 21.

Regions such as the Dalmatian coast are prospering – at the expense of ports further east in the Mediterranean – and the smaller Baltic states and some off -the-beaten-track Norwegian ports are muscling in.

Cunard themes include a genealogy cruise in association with Ancestry.com during a westbound Transatlantic crossing, on Queen Mary 2, and a photography cruise on Queen Elizabeth to visit the Arctic Circle and the midnight sun.

Cunard director Angus Struthers makes no mention of fares or discounts in his introduction to 2018. “Our programme offers an incredible richness and variety with itineraries and events that appeal to all,” he says. “Whether it’s a two-night break or a 24-night voyage, the Queens of Cunard offer every single passenger luxury on a grand scale.”

Royal Caribbean, meanwhile, says it is still finalising its 2018 itineraries. UK & Ireland managing director Stuart Leven says: “Our guests are booking their 2017 holidays in their droves and we are enjoying helping them find a truly extraordinary holiday for next year.”

It’s unlikely to happen with P&O or Cunard, but there is one risk with booking a holiday so far ahead. The ship might be switched to a completely diff erent itinerary.

Celebrity and Holland America Line have both taken ships out of planned Mediterranean programmes in 2017 – no doubt as a result of the falling numbers of American passengers prepared to travel to Europe.

Celebrity Equinox has been switched to the Caribbean, although the company will still have four ships – Constellation, Eclipse, Refl ection and Silhouette – in Europe.  Oosterdam has been redeployed to Alaska. Perhaps that’s why Royal Caribbean, playing safe,  is still keeping its 2018 customers waiting.