Campaigners are calling for action to reduce the amount of "hidden" fat in processed and fast foods.
Trans fats don't appear on nutrition labels
A survey by the consumer magazine Which? found high amounts of trans fats in a wide range of products.
Trans fats occur naturally in small amounts in dairy products and meat, but are also formed by the process used to extend shelf life of processed food.
The fats are thought to increase the risk of heart disease by raising levels of bad cholesterol.
The European Food Safety Authority has concluded that, gram for gram, trans
fats may cause more damage to the heart than saturated fats.
Trans fats are formed by a process known as hydrogenation, which is used by food manufacturers to extend the shelf life of their products. Hydrogenation turns the fat solid.
The current recommendations for trans fats are 4.4 grams a day for women and 5.6g for men.
A recent government survey estimated that the average adult intake is around 3g a day.
But Which? highlights research indicating that that just one gram of trans fats eaten daily over many years significantly increases the chance of heart disease.
Which? tested 30 popular foods which contain hydrogenated or partially hydrogenated fat or oil.
Highest was a portion of regular crispy strips and fries from Kentucky Fried Chicken which contained 4.4g of trans fat.
Highest levels of trans fat
KFC Colonel's regular crispy strips and fries - 4.4g/portion
McDonald's McNuggets and regular fries - 3.0g
Saxby's fresh ready rolled short pastry - 2.5g
Tesco Free From toffee fudge shortbread - 2.5g
Burger King Whopper and regular fries - 2.3g
Sainsbury's puff pastry minced beef & onion pie - 1.8g
Lidl Le Chef cheese, onion & chives quiche - 1.3g
Cadbury's Boost bar - 1.2g
Next was McDonald's McNuggets and regular fries, with 3g of trans fats.
Supermarket chain Tesco's toffee fudge shortbread contained 2.5g of the fat.
Malcolm Coles, editor of Which?, said: "It's irresponsible of food manufacturers to include an artificial ingredient in processed food that can only do you harm.
"If the government really is committed to improving the health of the nation, it urgently needs to take action on this disgusting stealth fat and give food manufacturers an ultimatum to stop peddling it.
"By the end of this year, we want targets set for manufacturers to eliminate artificial trans fat from their products.
"We also want information on trans fat to be included on nutrition labels.
"At the moment it's impossible to know how much trans fat we're consuming as it doesn't have to be labelled.
"As some experts believe the only safe level of artery clogging trans fat is
zero, this has got to change."
McDonald's said in a statement: "We cook all our products in 100% rapeseed
oil and since 1998 have reduced the trans fatty acid levels by over 50%.
"We continue to trial new oils that offer us further reductions."
KFC and Tesco were unable to comment initially on the results.
The Food and Drink Federation said: "The food industry is fully committed to reducing the level of trans fats to as low as is technically possible and has been actively reducing these levels.
"In fact, the government's latest national diet and nutrition survey shows
how the consumption of trans fats has fallen from 2.1% of total energy in 1985
to 1.2% energy in 2000."
The Food Standards Agency said it had no plans to take further action to persuade companies to reduce the use of trans fats.
June Davison, of the British Heart Foundation, said: "Trans fats have no nutritional value.
"It is isn't easy for consumers to know how much trans fat is in the food they eat, but the general rule is that if it contains hydrogenated vegetable oil you can assume it also contains trans fats."