The free upgrade is the Holy Grail of air travel. All the premium wine and legroom you can handle, without coughing up a small fortune for the privilege.

We’ve previously outlined in detail what passengers can do to increase their chances of scooping one (more below), but until now we'd never considered that a passenger’s dietary requirements would make any difference.

Nik Loukas, a former Qantas flight attendant and author of the plane food blog InflightFeed, claims that anyone with a special meal request - be it vegetarian, kosher or otherwise - can wave goodbye to their hopes of a move to business class.

Vegetarians might be saving the planet, but are they harming their chances of an upgrade? Credit: travelbook - Fotolia

"If there’s a special meal request in your booking, the airline won’t even look at you (for an upgrade)," he told Business Insider. "Because you’ve got a dietary requirement, and they might not be able to cater for you if they upgrade you."

Really? Telegraph Travel put the claim to six different carriers. So far only Qantas and Virgin have responded - and their comments should reassure all the veggies out there. "This isn’t true - dietary requirements have no impact on upgrades," says a Virgin spokeswoman. "If a customer orders a special dietary meal we can serve it in any different cabin."

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Qantas added: "This is untrue and the type of meal our customers order has no bearing on their ability to be upgraded. If the upgrade is awarded, the special meal request will be provided in their new cabin."

Surefire ways to improve your chances of moving up to the expensive seats include travelling alone, being a frequent flier with the airline in question – and simply being pleasant. This week Monarch said those who book over the phone and are "nice" to its representatives may be rewarded with an extra legroom seat and priority check-in.