Corrects throughout to show that weighting changes are by percentage points, not percent

Nasdaq will rebalance its benchmark Nasdaq 100 index next month, slashing Apple Inc's weighting and causing volatility as investors adjust their portfolios.

The rebalancing of the Nasdaq 100, a capitalization-weighted index, is not an unusual event in itself, but the timing of the move was a surprise even though many investors had noted Apple share's outsized influence.

The rebalancing will affect the relative weights of all the securities in the index and cause popular index-tracking funds such as the PowerShares QQQ to buy and sell shares to match the new composition.

In the short-term this is going to have a significant impact and create additional volatility, said Oliver Pursche, president at Gary Goldberg Financial Services in Suffern, New York.

The rebalancing hit shares of Apple and boosted shares of Microsoft Corp in premarket trading and was one reason for a fall in Nasdaq futures before the opening bell.

Nasdaq OMX is cutting the weighting of 82 securities to bring them more in line with their market capitalizations, the exchange said on Tuesday.

The extra weighting will then be distributed between the remaining underweight shares.

Apple will be by far most affected by the move, with an 8 percentage point cut in its weighting. Apple's shares fell 1.5 percent to $336.10 in premarket trades.

Much of Apple's excess weighting was distributed between other large cap technology shares that had become underwieghted relative to Apple.

Microsoft's weighting will rise 4.9 percentage points, sending its shares up 1.6 percent to $25.96 in premarket trade.

Other big changes include Oracle Corp, whose weighting will rise 3.4 percentage points, Intel Corp up 2.5 percentage points, Cisco Systems, up 2.1 percentage points and Google Inc up 1.6 percentage points.

The relative positions of the issues within the index will remain the same once the rebalancing takes effect May 2, according to Nasdaq.

The projected weight of Apple will be 12.33 percent of the index compared with a current weight of 20.49 percent, Nasdaq said on its website. Apple will continue to remain the largest component of index.

The Nasdaq 100 rebalancing will be based on index securities and shares outstanding as of March 31, the exchange said.

(Editing by Kenneth Barry, Padraic Cassidy, Leslie Adler)