FTSE slips 2 percent as Vodafone, banks weigh

* FTSE 100 falls 2 pct, snaps 3-day winning streak

* Vodafone slips 15 pct to 20-week low after trading update

* Miners gain tracking higher metals prices

By Atul Prakash

LONDON, July 22 (Reuters) - Britain's top share index fell 2 percent by midday on Tuesday, with banking stocks slipping on persistent concerns about tough economic outlook and heavyweight Vodafone (VOD.L: Quote, Profile, Research, Stock Buzz) slumping after a trading update.

By 1117 GMT, the commodity-heavy FTSE 100 .FTSE was down 109.6 points at 5,294.7, after closing 0.5 percent higher on Monday. The index, which rose in the previous three sessions, has fallen nearly 18 percent this year.

Vodafone topped the FTSE 100 losers' list by slipping 15 percent to a 20 week low, shaving nearly 48 points off the index, after it cut its revenue outlook as tougher economic conditions forced consumers to hold back from buying and using new handsets.

The company, the world's largest mobile phone firm by revenue, said full-year revenue was expected to be around the bottom of its previously stated range.