UK's FTSE has worst day so far in 2015 as BG and Pru fall

* FTSE 100 falls 2.5 pct to 6,702.84 points

* FTSE has worst 1-day fall so far in 2015

* Prudential CEO leaves for Credit Suisse (NYSE: CS - news)

* Tullow, BG renew oil price-linked slide

* Possible sale of securities arm lifts Charles Stanley (LSE: CAY.L - news)

By Sudip Kar-Gupta

LONDON, March 10 (Reuters) - Britain's top share index suffered its biggest decline so far this year on Tuesday, hit by drops in insurer Prudential (SES: K6S.SI - news) and energy stocks such as BG .

Prudential fell 3.1 percent as the loss of its chief executive, Tidjane Thiam, to Credit Suisse took the shine off a 14 percent rise in 2014 operating profits.

Energy stocks also slumped as oversupply and weak demand pushed Brent crude oil futures down. BG dropped 7.4 percent, Tullow Oil (LSE: TLW.L - news) 7 percent.

The blue-chip FTSE 100 index ended down 2.5 percent at 6,702.84 points. The FTSE has retreated from the record high of 6,974.26 points it reached on March 2. It remains up by 2 percent since the start of the year.

Engineer Weir Group fell 3.3 percent, reversing some recent gains. It had rallied nearly 12 percent since last Thursday on speculation that Weir, hurt by cutbacks in by oil firms, might attract interest from U.S. private equity firms.

On a brighter note, shares in financial services group Charles Stanley -- which is not in the main FTSE 100 index -- rose 5.4 percent after the group said it might sell its Charles Stanley Securities arm.

Some investors have said uncertainty leading up to Britain's general election in May may hinder the FTSE and prevent it from reaching record highs around 7,000 points.

Gerren O'Neill, senior trader at Thames Capital Markets, said the FTSE could drift lower in coming weeks but find support around 6,700 or 6,600 points.

"We continue to see further downside from here, but any pullback should be relatively short-lived," he said. (Additional reporting by Alistair Smout; Editing by Larry King)