Oxford's Mara Yamauchi was forced to pull out of the Olympic marathon after less than 10 kilometres after suffering from a bruised ankle.
She tearfully stopped at the side of the road 9.4km into the race after deciding she could not go any further.
Britain's medal hope Paula Radcliffe was forced to withdraw with a foot injury earlier in the week. The race was won by Ethiopian's Tiki Gelana.
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There is a high risk from birch pollen during any good weather. Oak pollen is also on the increase.
Wednesday's weather for the west of the region
Wednesday's weather for the east of the region