McDonalds are being accused of targeting unhealthy food at young people with plans for a bigger Big Mac.
The Big Mac is to be 40% bigger for the offer
Liberal Democrat health spokesman Steve Webb is starting a petition in Parliament saying there is no need for the 40% bigger burger.
He criticised McDonalds for using England's football World Cup campaign to promote the burger. The company says it is giving fans a special offer.
Mr Webb's attack came as Tony Blair urged people to live more healthily.
The bigger Mac is due to go on sale in June despite the company's previous moves away from "super size" portions.
Mr Webb said he was dismayed by the news, especially as he spent the weekend watching Super Size Me, Morgan Spurlock's film about spending a month eating only McDonald's meals.
The current Big Mac weighs 219g, contains 10g of saturated fats and 560 calories.
"The current Big Mac contains half the recommended daily intake of saturated fats - the bigger Big Mac will be getting on for three-quarters," said Mr Webb.
"Adults can make their own minds up - I eat burgers from time to time so I'm not saying they are evil.
"But McDonalds are targeting kids and sport as a great way to get through to kids."
The MP said record figures on childhood obesity in recent weeks showed a "health time bomb" was being created for today's youngsters.
He predicted his Commons motion saying "there is no need for a bigger Big Mac" would strike a chord and win support from other MPs.
Responding to his criticism, a McDonald's spokesman said: "The Bigger Big Mac promotion is a limited edition offer during the month of June offering football fans a little bit more of what they love during the World Cup."
The company has also moved to provide more salads and other options after making a massive loss in 2003.
The prime minister on Tuesday trumpeted a new government campaign to encourage people to make small changes in their lifestyle.
He promised to take the stairs rather than the lift and to drink as much water as tea.
His spokesman said Mr Blair was also still using his rowing machine but he refused to comment on the Big Mac plans.
"We have to give people the information to make informed choices, we cannot force people to live healthy lifestyles," he added.
Public Health Minister Caroline Flint said: "We all know that we should eat more fruit and vegetables and get more exercise to improve our health but sometimes improving our own health can be daunting.
"There are everyday, simple choices people can make in their lives which will have a direct impact on their health.
"Eating an extra piece of fruit or walking up the stairs can help people add years to their lives."