Cheapest flight tickets? Book 59 days before departure

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Fluctuating flight costs can make booking airline tickets frustrating, but travel website momondo.co.uk claims to have identified the optimal time to get the best bargain.

Contrary to popular opinion, travelers won't benefit from booking tickets a year in advance, nor from leaving it until the very last minute, momondo says. On average, the ideal time to book is 59 days prior to departure.

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Momondo said it tested its own website's 13 most popular flight routes out of London -Copenhagen, Paris, Berlin, Lisbon, Dublin, New York, Amsterdam, Barcelona, Madrid, Istanbul, Edinburgh, Glasgow and Los Angeles - as part of its research.

It found that travelers could save anywhere between 18 and 61 percent on airline fares - 38 percent on average - by booking 59 days prior to departure. The savings comparison was made to the most expensive day to book tickets - usually the day of departure.

Finding the cheapest price two months in advance was a uniform trend, momondo found, despite the varying flight times and routes - both domestically and internationally.

The cheapest fares were found by booking between 54 and 66 days in advance of departure, resulting in the average of 59 days.

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However, momondo acknowledged that along with the date of booking, the cost of an airfare was also influenced by factors like date of departure, time of day, season and airports of departure and arrival.

"We are pleased to uncover some new trends around booking flights, proving it is always cheapest to book on average two months ahead of travel," said Julie Pedersen, spokesperson for momondo. "So travelers need not book six or 12 months in advance, to find the best airfares."

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—By CNBC's Katie Holliday: Follow her on Twitter @hollidaykatie