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* FTSE 100 down 0.5%, FTSE 250 off 0.3%

* Sainsbury falls after maintaining annual financial guidance

* Energy shares gain on higher oil prices

* British election in focus

July 2 (Reuters) - London's FTSE 100 tumbled on Tuesday after ending the previous session flat as investors maintained caution ahead of the parliamentary elections at home, while an uptick in oil prices contained declines.

The blue-chip FTSE 100 was down 0.5%, hitting its lowest in more than two weeks, while the mid-cap FTSE 250 was off 0.3% at 0715 GMT.

Investors refrained from placing big bets ahead of the UK parliamentary elections on Thursday, where the Conservatives look set to be replaced after 14 years.

Meanwhile, prices in British shops rose at the slowest pace in almost three years last month, according to industry figures that underscore how inflation has cooled.

The last key inflation reading showed that May inflation had fallen to the Bank of England's 2% target for the first time in nearly three years. However, investors do not expect it to help Prime Minister Rishi Sunak's fortunes in the poll.

GSK slipped 1.2% after a Delaware judge rebuffed a request by the drugmaker and others to appeal a ruling allowing over 70,000 lawsuits claiming that the heartburn drug Zantac caused cancer to go forward.

Sainsbury's, Britain's second largest supermarket group, slipped 1.5% after it kept its annual financial guidance as it reported a 3% rise in first-quarter underlying sales.

The stock weighed on the personal care, drug and grocery sector that fell 1.3%.

Energy shares were the top gainers after oil prices held near two-month highs on expectations of rising fuel demand from the summer travel season and possible U.S. interest rate cuts.

Heavyweight oil giant BP gained over 1%.

A job openings report is due in the United States later in the day, alongside Federal Reserve Chair Jerome Powell's speech. (Reporting by Purvi Agarwal in Bengaluru; Editing by Mrigank Dhaniwala)