Many contain high levels of sugar and some are promoted for use from four months of age - a time when it is recommended that babies should still be on a diet of breast or formula milk.
Babies would need to eat twice as much shop-bought food to get the same energy and protein as meals cooked at home, researchers found.
The study, from the department of human nutrition at the University of Glasgow, said that many weaning foods "would not serve the intended purpose" of giving a baby extra nutrients or a range of tastes and textures.
Current guidelines encourage weaning from six months of age, but baby foods are often labelled as "suitable from four months".
Experts analysed all the baby foods produced by the main UK manufacturers: Cow & Gate, Heinz, Boots, Hipp Organic, Ella's Kitchen and Organix. Products included ready-made soft foods and dry foods such as cereal, biscuits, rusks, bars and cakes.
Of the 479 items, 364 (79%) were ready-made spoonable foods and 201 (44%) were aimed at infants from four months. Some 65% of the products were sweet foods.
The researchers said the typical calorie content of spoonable foods was 282kJ per 100g, almost identical to breast milk at 283kJ per 100g.
Foods made at home - such as chicken stew, beef with mash, and stewed apple with custard or rice pudding - were "more nutrient dense" than shop-bought foods.
And while commercial finger foods had more calories, they had a "very high" sugar content.
The iron content of most of the foods was also lower than that found in formula milk.
The researchers concluded: "Most products are ready-made spoonable foods that are no more energy dense than formula milk, and are generally much less nutrient dense than homemade foods."
A statement from Heinz said its products were "specially prepared to meet babies' nutritional needs, while Organix said it did not add vitamins and minerals to foods due to organic production rules, unless they were required by law.
Nobody was available for comment from Hipp Organic.
Helen Messenger, from Cow & Gate, said: "Our foods offer good quality nutrition."
Rosemary Dodds, from the National Childbirth Trust, said manufacturers were "dragging their feet" in responding to new thinking that babies don't generally need solids before six months.