Holmes, the 2004 double Olympic Gold medallist, spoke to Marketing during Jaguar's Academy of Sport event, a mentoring and education programme that provides support to young athletes. Holmes works as a brand ambassador to Jaguar.
Questioned about the involvement of sponsors, including fast-food and drinks companies, in sport, Holmes said: "Every sport is quite different. But generally, it is quite expensive to be involved in sport.
"Everybody who starts young, like me at the age of 14, wants to be Olympic champion.
"It is critical to get the support of brands. Athletes are under a lot of psychological pressure, so the last thing they need is the pressure of not being able afford to compete. That is where sponsors come in.
Holmes also made the point that Coca-Cola and McDonald's, two Olympics sponsors which some believe should be banned from sponsoring major sporting event, are responsible for some vital investment in athletics.
She said: "These companies put money into helping fund grassroots sports."
McDonald's works with the Football Association on grassroots football, supporting coaches and volunteers, while Coke has also been involved in working with grassroots sport.
Holmes' remarks follow a series of media reports criticising London 2012 sponsors including McDonald's, Coca-Cola and adidas.
Doctors' claims that brands like McDonald's and Coca-Cola should be banned from sponsoring the Olympics were labelled as "unfair" and "a serious abdication of their responsibility", by the voice of British advertisers, ISBA earlier this week.
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