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The planned sugar tax will "hit poorest families hardest" and has nothing to do with the sugar content of products, the Taxpayers' Alliance says.

The group, which wants the levy to be axed, tested 49 drinks and found that some coffee shop drinks had more sugar than Coca Cola, but would not be taxed.

Anti-obesity campaigners welcomed the tax when it was announced in March.

The Treasury said soft drinks would be taxed because they were the main source of added sugar in children's diets.

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The Taxpayers' Alliance (TPA) survey found that Coca-Cola, with 10.6g of sugar per 100ml, will be subject to the levy, but a Starbucks signature hot chocolate with whipped cream and coconut milk, which has 11g of sugar per 100ml, will not.

The study also noted energy drinks such as Monster Origin, 11g/100ml, will be taxed, but Tesco chocolate flavoured milk, 12.4g/100ml, will not be.

'Regressive tax'

Overall, the 10 most sugary drinks analysed by the group which campaigns for lower taxes will not be subject to the levy.

The recommended maximum intake of added sugar per day for those aged 11 and over is about 30g or seven teaspoons, the NHS says.

TPA chief executive Jonathan Isaby said it was "deeply concerning" that the government was "pushing ahead with this regressive tax which will hit the poorest families hardest".

"The evidence shows that the sugar tax has nothing to do with the sugar content of products, so it is farcical to suggest that this will have any positive impact on people's diet or lifestyle choices," he said.

"This is yet another example of irresponsible meddling from the high priests of the nanny state, introducing entirely unnecessary complications into an already complicated tax system and pushing up the cost of everyday products for hard-pressed families."

The survey's 10 most sugary drinks

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Costa, chai latte, skimmed milk (17.5g/100ml)

Caffe Nero, hot chocolate, no cream, regular (17g)

Galaxy flavoured milk (14.4g)

Starbucks, white chocolate mocha with whipped cream, short, skimmed milk (13.3g)

Tesco chocolate fudge brownie flavoured milk (13.1g)

Frijj chocolate fudge brownie milkshake (12.9g)

Mars flavoured milk (12.8g)

Tesco chocolate flavoured milk (12.4g)

Caffe Nero, frappe creme, strawberry and vanilla (12.2g)

Galaxy thick shake (11.6g)

The NHS has also said it would impose its own "sugar tax" in hospitals and health centres in England.

Chief executive Simon Stevens said he wanted the 20% tax on all sugary drinks and foods in NHS cafes to be introduced by 2020.

It is hoped the NHS levy, which would initially just apply to sugary drinks, could raise £20m-40m a year, with the money raised used to improve the health of its workforce.

The "tax" would also be put in place with the aim of discouraging staff, patients and visitors from buying sugary goods.

'No nutritional value'

A Treasury spokesman said the soft drinks industry levy was "a major step forward in our efforts to tackle childhood obesity".

"Treating obesity and its consequences costs the taxpayer £5.1bn every year," he said.

"The levy will be charged on soft drinks because they are the main source of added sugar in children's and teenagers' diets, many with no intrinsic nutritional value.

"Health experts agree there is a specific problem with sugar-laden fizzy drinks that must be addressed."

He said the money from the tax would go towards funding more school sport and expanding school breakfast clubs.