The FTSE 100 Index fell 117.9 points, or 2.1%, to 5533.9 amid worries that a raft of Spanish regions were poised to ask for government bailout funds, increasing the likelihood that the country itself will turn to the EU for help.
The fall wiped nearly £30 billion from the value of London's leading shares index, with all 100 companies losing value and banks falling up to 4%.
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Mr Blavatnik, whose empire includes the Warner Music Group, has an estimated fortune of £13.17bn according to the Sunday Times Rich List.
Find out what to expect from the weather if you are running in the big race in London or watching from the sidelines.
It will be a soggy start to the weekend and temperatures will drop making it feel colder than in recent days.