It's the classic traveller's dilemma. When is the best time to book a flight for the cheapest deal? Do you bag a flight as early as possible or risk leaving it until the last minute? The answer, according to flight comparison website Skyscanner, is to wait, but not for too long.

After an epic piece of number crunching of two million flight searches over three years Skyscanner has come up with the ideal time to book 10 summer sun destinations. Flights to Spain, for example, are at their cheapest (£153 for a return) five weeks before departure, while the cheapest flights to Greece (£228 return) are available just three weeks ahead.

However, Filip Filipov of Skyscanner advises that while five weeks is the average, the optimum time to book your flight can vary significantly between destinations. "Price patterns can vary by type of airline and between short and long haul travel, so is very much destination dependent."

The best time for flights to Turkey, for example, is 13 weeks before departure, while the US should be booked five months (21 weeks) ahead.

Skyscanner's research covers scheduled and low-cost flights (charter flights represented less than 1.5%), and is contrary to the advice given by most industry experts who say, when it comes to full-service scheduled flights, the golden rule is the earlier the better.

"Generally flight prices go up rather than down the closer you get to travel, so the earlier flights are booked, the better," says Andy Washington, managing director of Expedia.

Washington's advice is that if you have a specific destination and specific dates in mind, the safest option is to book as soon as possible after seats go on sale, which for most scheduled airlines is between 10 and 12 months in advance (see the examples below). Most scheduled airlines hold at least one major sale in a year, usually in January, so now is also a good time to look for bargains outside of peak travel times.

Charter airlines, by contrast, may reduce their prices closer to departure if they are struggling to sell seats, so it can be worth waiting to book – but only if you don't have a specific destination in mind as there is no guarantee that they will be discounting the place you have your heart set on.

Here is a list of Britons' favourite summer destinations, with the average return flight price for summer and the best time to book (weeks before departure).



Below is a selection of airlines with details of when fares go on sale, and the best time to pick up a bargain.

up to 11 months in advance.January and September.London Heathrow to Auckland, Wellington or Christchurch via Shanghai from £855 return for travel between 1-31 March 2014.

Virgin Atlantic

Seats released for sale 11 months in advance

Sale period January

Sample sale fare Heathrow to New York from £428 return (must be booked by 28 January for travel by 31 March).

Singapore Airlines

Seats released for sale up to 12 months in advance.

Sale period started 20 December 2013 and runs until 28 February.

Sample sale fare Heathrow to Singapore from £640 (for travel between 14 January-11 April; 27 April-24 June 2014; and 17 August-4 December).

Qantas

Seats released for sale 353 days in advance

Sale period year-round apart from peak travel times of July/August and December/January.

Sample sale fare Heathrow to Sydney from £826 for departure on 1 Februaru-15 March and 17 April-30 June. Book by 31 January 2014.

BA

Seats released for sale between 350 and 355 days in advance.

Sale period January and September.

Sample sale fare Heathrow to Rio de Janeiro from £647.25, saving £163, for travel before the end of March 2014.

Monarch Airlines

Seats released for sale up to 14 months in advance.

Sale period occasional flash sales.

Sample sale fare start at £29.99 one-way on winter routes to £39.99 one-way for summer routes (book by end of January for departures until end of June).

CHARTERED AIRLINES

Flythomascook

Seats released for sale Up to 20 months in advance.

Sale period Depends how well seats are selling but there are promotions throughout the year. The current sale runs January until 21.

Sample sale fare Manchester to Corfu from £197 return for seven nights, departing 6 June, including baggage and meals.

LOW-COST AIRLINES

Ryanair

Seats released for sale 8-12 months in advance.

Sale period weekly.

Sample sale fare London Stansted to Malmo, Sweden, from £16.99 one-way for travel in February and March.

Easyjet

Seats released for sale six to nine months in advance.

Sale period January, with other promotions throughout the year.

Sample sale fare up to 20% off 85,000 seats for travel between 5 February and 2 April (sale ends 21 January). London Gatwick to Copenghagen from £26.49 one way for travel in March.