The power of the politicians in the leaders' debate may be boosted by having the best body language on stage.

It may not be the sort of thing that might trigger a landslide victory but in the UK's close-run general election it could tempt a few floating voters, according to body language expert Robert Phipps.

He said: "Body language is only going to help with the undecided voters. Those who have already made their minds up to go with one shade of politics or the other are going to stick with that unless their leader gives a particularly bad performance.

"It is the undecideds - the 1-2% of votes (going) one way or the other which in this election is looking to be so close that they could be crucial.

"It is giving them an understanding of who they are voting for, who is giving the best performance on the night regardless of what their politics are."

One of the strongest examples of the power of body language came at the dawn of the television age in the 1960 US presidential debates where a young Senator John F Kennedy, the Democratic candidate, faced up to Republican candidate Vice President Richard Nixon in the race for the White House.

More than 60 million Americans tuned in to watch the first-ever televised debate and among television viewers Mr Kennedy was regarded the outright winner, while those that heard it on the radio thought Nixon had won.

Mr Phipps said that Mr Kennedy's image was "sharp, clean shaven" while Mr Nixon appeared to have a "five o'clock shadow, looked very dark, looked worn out, looked tired".

This heralded an age where politics has been dominated by image, sound bites and short little bursts to get a message across, according to Mr Phipps.

"The more they can use their bodies to get their message across the more powerful it is, the more emphasis it has."

Now viewers can tune in to ITV's two-hour televised general election debate featuring seven leaders who have been given colour-coded podiums reflecting their parties. The order they will appear on screen is Green, Liberal Democrat, Ukip, Labour, Plaid Cymru, SNP, Conservative. The debate is also being shown live on the BBC News channel and streamed on the online election live page.

Tell-tale tricks to look out for, according to Mr Phipps, is how often they grip the side of the lectern as this can show the tension they are feeling or that they are wanting to express themselves. They should also watch out for how many times the politicians turn their shoulders and whether it is toward the person they are talking to or if they are using it and their whole body to try and drive a message home.

Keen-eyed viewers may also spot "micro-expressions" in the faces of the debaters.

Mr Phipps notes they "last less than a quarter of a second but are the true emotions that have been felt". It can range from a momentary wrinkle of the nose or snort of disgust which shows they disprove of something through to trying to cover up fear that is obvious from an initial grimace but is followed by a smile.

On the art of presentation Mr Phipps describes David Cameron as someone who is "very expressionless" and who has very few lines on his face while Ed Miliband is "much more engaging" and gets his whole body involved. He looks at the person who is asking the question and answers them directly.

He also points out that Nick Clegg had been particularly good at stepping away from the lectern in the 2010 election debate which meant that people could see his whole body. It meant that when he was putting a message across there was much more emphasis.

Mr Phipps noted: "This time it will be different because he will be hemmed in on either side and won't be able to express himself as much."

He added that not only will he have to contend with this but he has been in government for five years with the Coalition so he is going to have to defend his position.