Two popular soft drinks, Lucozade and Ribena, are to be reformulated to contain less sugar, public health minister Anna Soubry has announced as part of the "responsibility deal" the government has agreed with food manufacturers.

The deal was hailed by Soubry and the food industry as evidence that the government's voluntary approach to tackling obesity is paying dividends.

But campaigners were unimpressed by the sugar and calorie reductions, announced at a Food and Drink Federation (FDF) event in London.

A 380ml bottle of Lucozade Energy – the original recipe for the drink first launched in 1927 – contains 33g of sugar and 266 calories. The planned reduction of 8% in its sugar content and 9% in calories will not make much difference to those who are overweight and in the habit of drinking it, they say. A 500ml bottle of Ribena Blackcurrant ready-to-drink contains 52.6g of sugar and 216 calories. The reformulated product will contain 10% less sugar.

Charlie Powell of the Children's Food Campaign pointed out that in spite of the change, the drinks would still qualify for a red traffic light under Food Standards Agency guidance, because of sugar levels above 6.3g per 100ml. "So they are swapping one red light for another: not exactly impressive," he said.

Both drinks are owned by GlaxoSmithKline, Britain's biggest pharmaceutical company, which said that, as a healthcare company, "we have a responsibility to help people make healthier choices." A spokeswoman added that, "we are saying this is a first step. It is the start of a journey."

Some other popular drinks are also to contain fewer calories, Soubry said.

AG Barr, which produces IrnBru, will reduce the calorific content across their portfolio of drinks by 5%. J2O will launch two new products with 10% fewer calories than the standard 275ml bottle.

But Powell said sporadic announcements of voluntary agreements were not enough. "Lots of companies making tokenistic commitments does not constitute effective action to tackle obesity – it is regrettable that the Department of Health is pushing a smoke-and-mirrors strategy which puts public health behind commercial interests," he said.

AG Barr, he said, had committed months ago to removing two food colourings (E110, Sunset Yellow and E124, Ponceau) from its products after EU warnings that they can make children hyperactive. But nothing had happened, he claimed. "Where is the responsibility there?" he asked, saying that it seemed to be "a perfect example of the lack of enforcement in a voluntary approach".

A spokesperson for the company said the commitment stood. "With IrnBru the programme continues to go well, however with such a unique product we'll only complete the process when we are 100% satisfied that the outcome is right and there is no compromise for our consumers. In the meantime there is a temporary declaration on packs giving consumers information about the issue."

Co-Operative Food and Burton's Biscuits also made pledges on reducing the calories in their products. They join 23 companies that have now made pledges relating to healthier food and drinks.

Soubry said that more companies and supermarkets need to join them.

"Through the responsibility deal we are already achieving real progress in helping people reduce the calories and salt in their diet. Overall, more than 480 companies including many leading high street brands have signed up.

"All of the major supermarkets have now committed to removing artificial trans fats, and over 70% of fast food and takeaway meals sold on the high street have calories clearly labelled, but more needs to be done.

"We are encouraged by the extra businesses which have signed up today but I want to see even more progress. All in the food industry have a part to play and I now expect companies which are not yet taking action to come forward and make pledges."

Soubry and the FDF said real progress had also been made on cutting salt in food, citing a "10% reduction in the salt levels of products made by FDF's members when compared to 2008". But campaigners Cash (Consensus Action on Salt and Health) pointed out that the target was for a 15% reduction on 2010 levels.

"Clearly more work can be done by the FDF, and the government, to increase sign-up of their members to the Department of Health's salt reduction pledge, and to share best practice for salt reduction amongst poorer performing companies," said Katherine Jenner, campaign director.