UPDATE 1-Most UK retailers' own-brand soft drink prices will not rise after sugar tax

* Tesco, others reformulated entire fizzy drink line

* Sainsbury's says wants to give choice to shoppers

* UK sugar tax to come into effect on April 6(Adds comments from other chains, bullet points, byline)

By Martinne Geller and Costas Pitas

LONDON, April 3 (Reuters) - Britain's biggest supermarketchain Tesco said its own brand fizzy drinks will not goup in price when a sugar tax comes into effect this week as theyare already below a government-imposed threshold.

Fizzy drinks made by rival supermarket chains Asdaand Morrisons will also be unaffected, as will manybranded drinks, as their sugar content has been reducedsufficiently to be exempt from the levy. Soft drink makers havehad two years to prepare for the tax, although there will beprice increases for some own-label drinks made by Sainsbury'sas well as Coca-Cola Classic.

Britain will implement the tax on makers of sugary drinksfrom Friday, a move advocated by health campaigners who arguethe beverages are a source of empty calories.

A charge of 18 pence ($0.25) per litre will be levied ondrinks containing 5 grammes or more of sugar per 100 millimetresand 24 pence per litre on drinks containing 8 grammes or more.

The tax was announced in 2016, giving companies time tochange their recipes.

Tesco said it had spent years cutting the level of sugar inits own label products and will not be subject to the levy. Allof its own-brand soft drinks fell below the levy's threshold in2016.

Asda and Morrisons have also worked recently to reformulatetheir entire lines of carbonated soft drinks, according tocompany officials.

Of Britain's big four supermarket chains, only Sainsbury'shas not reformulated its entire range.

"Health matters to our customers and reformulation has beena fundamental part of our approach to healthy eating for manyyears," a spokeswoman for Sainsbury's said, noting the companyhas removed more than 2,667 tonnes of sugar from its own brandsoft drinks. She added however: "We think it's important thatcustomers have choice, which is why we continue to offer a fullrange of soft drinks, including no added and reduced sugaroptions."

Tesco, which has a 28 percent market share in Britain andalso operates abroad, said 85 percent of the drinks sold in itsstores will be exempt from the charge although some brand priceswill rise.

Coca-Cola, for example, has reformulated the bulk ofits portfolio, except for its flagship Coca-Cola Classic, itstop-selling brand, which should see its price go up if retailerspass the levy on to consumers.

UK drinks bottler Britvic said 72 percent of itstotal portfolio is already below the threshold. Excluding thePepsiCo brands it sells, 94 percent is exempt, aspokeswoman said.

But cuts in sugar, including to soft drinks Irn Bru, knownas Scotland's second national drink, have prompted outrage amongsome fans.

Confectioner Nestle also said last month it waslaunching a lower-sugar Milkybar in a bid to partly address oneof Big Food's toughest challenges - how to make junk foodhealthy but keep it tasty.

The "war on sugar" being waged by governments and consumersto combat public health emergencies like diabetes is slowinggrowth in global demand, challenging countries such as Braziland India that are reliant on the sector.(Reporting by Costas Pitas and Martinne Geller; Additionalreporting by James DaveyEditing by Stephen Addison and Susan Fenton)