By Mary-Ann RussonBusiness Reporter, BBC News

French food giant Danone is looking into reports that its Aptamil baby milk formula is making some infants ill.

Aptamil recently changed the recipe of three of its baby milk formulas.

Hundreds of parents in the UK have complained on social media that the revised formula milk is making their babies sick.

Danone said it had carried out extensive safety checks, but added that it was "taking all feedback very seriously".

The food giant is now investigating the complaints and has set up a free helpline that parents can call for advice.

Parents in the UK are advised to call 0800 996 1000, pressing option 0 followed by option 1.

There have also been complaints that the size of the revised products has been reduced from 900g to 800g a tub, but the price remains about £11.

Several mothers have told the BBC that their babies experienced upset stomachs after they started using the new version of Aptamil First Infant Milk powder (stage 1).

The parents said they noticed that the milk powder did not dissolve in the bottles, leaving clumps of residue.

"It smelled different, like gone-off milk that had been left out for a day," Rosie Menzies, from Lymington, Hampshire, told the BBC.

She said that her 11-week-old son Harry would be sick soon after feeds and began to refuse bottles, until she started using a different brand.

Danone UK said: "We would like to reassure parents that the quality and safety of our products is our number one priority. We have recently introduced new Aptamil formulations and we recognise that some families have not found the transition to the new formula easy.

"We have undertaken extensive quality and safety checks, including clinical trials, product testing and product experience tests on these products. The results have shown that babies take to this formula well and that it is safe.

"We have updated the mixing instructions on our packs as this new formula requires parents to mix it up slightly differently, compared to the previous formulation - specifically, shaking vigorously for 10 seconds to dissolve the powder."

Aptamil said it had changed the recipes of three products: Aptamil First Infant Milk powder (stage 1), Aptamil Follow On Milk powder (stage 2) and Aptamil Growing Up Milk powders (stages 3 & 4).

'Scientific research'

When asked why the baby milk formula was changed, Danone did not immediately reply.

Rebecca Heal in Larkfield, Kent, told the BBC that she complained to Aptamil about the new recipe and smaller packaging, and was told that the formula was changed because of "scientific research and costs to cover that".

Ms Heal, who has a three-month-old daughter Olivia, said that she tried to follow Danone's new mixing instructions, but the milk still wasn't right.

"It took a lot of effort and shaking to get it to mix properly," she said.

"Even after a minute or so of vigorous shaking, the milk still had gritty residue on the sides of the bottle, but just increased the amount of froth on the top as a result."

However, some parents using the revised formula posted on Aptamil's Facebook page that they had not experienced any problems.

One mother, who did not want to be named, told the BBC that her four-month-old baby had successfully transitioned from breast milk to Aptamil First Infant Milk.

"There is some residue after mixing, but then I found this was was often the case with the old formula anyway," she said. "Coming from a science background, I'd rather look at things analytically, than just jumping on hysteria."

Courtney Wheeldon, based in Hull, said that her 17-week-old son Jenson, who was born prematurely, had benefited from being on the new revised formula.

"My son appears a lot more content and happy... he's putting on a lot more much-needed weight," she told the BBC.

"My only complaint about the new formula would be that there is 100g less in the tubs for the exact same price."

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