A record high for the FTSE 100
It reached 6105.8 in early trading on Friday before a dramatic fall of 200 points in the space of a minute.
The fall temporarily wiped £34 billion off the value of shares in London.
Despite the downward spiral, at midday the index had recovered to around 6039.
New trading system
The volatile market was put down to the effect of the introduction of a new high-speed electronic trading system called SETS.
SETS speeds up share dealing, but according to analysts can lead to dramatic swings in the market.
Friday morning's see-saw trading came as dealers frantically bought and sold shares as they attempted not to lose out on highly complicated stock market bets known as futures options.
Jeremy Batstone, head of research at NatWest Stockbrokers, said: "They must have a quirk on the new Sets system on the back of traders squaring their positions on future options.
"The liquidity in the market is different to what it was, the orders are driving the market and this is making the Index so volatile."
The overall upward trend of the London market was prompted by Thursday's figures from the Bank of England showing a larger than predicted growth in deposits at banks and building societies.
A bullish finish to trading in the US and the Far East also encouraged dealers.
The FTSE closed 41.6 points down at 5956.3.