Storing donated blood too long increases the chance of an infection, US researchers claim.

The risk of blood poisoning or pneumonia doubled once the 29-day mark passed, Cooper University Hospital in New Jersey found.

The study, presented at a US conference, calls for increased care over the way blood is used and stored.

UK authorities said blood took 10 days on average to reach hospitals and daily deliveries meant it was not stockpiled.

The US does not allow the use of blood stored for longer than 42 days - in the UK this is lower, at 35 days.

After two weeks in storage, red blood cells start to undergo changes which lead to the release of chemicals called "cytokines".