By Rory Cellan-Jones BBC industry correspondent

Rival supermarkets both claim their schemes are influencing shoppers

If you care about what kind of food you buy, there is suddenly a lot more to digest.

The big supermarkets are putting new labelling systems with more nutritional information on the front of packets - but what you get depends on where you shop.

Sainsbury, Asda and Waitrose have all opted for a traffic-light label, where green is good, and red is a warning not to consume too much.

This is the system the Food Standards Agency would like the whole industry to adopt.

But Tesco and many of the big food manufacturers have gone for the rival Guideline Daily Amounts (GDA) system, where the label tells you what percentage of your guideline daily amounts of sugar, salt, fat and calories each serving contains.

Each side insists its system is working, and influencing the way customers shop.

Shopping habits

Tesco points to a 33% drop in sales of a shredded chilli beef ready meal with high levels of salt, compared to a 16% rise in sales of Chicken Chasseur, a meal with far less salt.

"What they seem to be doing," says Lucy Neville-Rolfe, corporate affairs director at Tesco, "is using the system to say I need to cut my salt."

She says traffic light schemes can be too crude: "For instance, high-fat cheese and low-fat cheese both turn out to be red, so you can't make that crucial choice."

Sainsbury's claim their wheel of health system is easier to use

But over at Sainsbury's they too have research proving their "wheel of health" label works.

Sales of a pepperoni pizza with lots of red on the label are down 10%, while the healthier Hawaiian pizza is up by the same amount. And guess what - they are not impressed by Tesco's GDA scheme.

"Our research found 60% of customers don't understand percentages so it's extremely challenging to shop in the store in an informed manner," says Beth Flower, nutrition and health manager at Sainsbury's.

"We've taken the worry away by giving some simple colour-coding."

Traffic lights

So with no agreement on what works best BBC News set out to do its own research.

At Enfield shopping centre north of London, BBC News asked shoppers to examine the two types of label and say which was most helpful if they were worried about their health.

A hundred people took part - by no means a scientific survey but a useful snapshot - 67 favoured the Sainsbury's "wheel of health", with 31 going for Tesco and two undecided.

Manufacturers Kellogs and Nestle are backing the GDA system

A pretty clear victory for traffic lights, but then we showed the shoppers two ready meals from each supermarket and asked them to identify the healthiest in each case.

This time both systems seemed to work - 90 of the 100 who took part correctly identified the healthier of the two Tesco meals, while 83 got it right using the Sainsbury "wheel of health".

The Food Standards Agency says it would be simple enough to add traffic lights to the GDA label and it is appealing to the food industry to unite behind once scheme.

But with Tesco planning to put GDA labels on 6,600 of its own products, and manufacturers like Kellogs and Nestle backing its approach and resisting traffic lights, the supermarket giant seems to be winning the label war.

You can watch the full Rory Cellan Jones report on the Six O'Clock News on BBC One at 1800 BST on Friday