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From the UK Food Standards Agency:

Use by

You will see 'use by' dates on food that goes off quickly, such as smoked fish, meat products and ready-prepared salads.

Don't use any food or drink after the end of the 'use by' date on the label, even if it looks and smells fine. This is because using it after this date could put your health at risk.

For the 'use by' date to be a valid guide, you must follow carefully storage instructions such as 'keep in a refrigerator'. If you don't follow these instructions, the food will spoil more quickly and you may risk food poisoning.

'Use by' does not always mean 'eat by'. If a food can be frozen its life can be extended beyond the 'use by' date.

But make sure you follow any instructions on the pack - such as 'freeze on day of purchase', 'cook from frozen' or 'defrost thoroughly before use and use within 24 hours'.

It's also important you follow any instructions for cooking and preparation shown on the label.

Once a food with a 'use by' date on it has been opened, you also need to follow any instructions such as 'eat within a week of opening'.

But if the 'use by' date is tomorrow, then you must use the food by the end of tomorrow, even if you only opened it today.

hth

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Note added at 12 hrs (2008-02-13 16:14:00 GMT)
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see: http://homecooking.about.com/library/weekly/aa102102a.htm