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* FTSE 100 up 0.5%, FTSE 250 off 0.1%

* British election polls underway

* Smith & Nephew gains after Cevian Capital discloses stake

July 4 (Reuters) - London's FTSE 100 climbed on Thursday, boosted by Smith & Nephew, while parliamentary polls in the country were underway, that the Labour Party looked set to win.

The blue-chip FTSE 100 was up 0.5% after logging its best day in nearly two weeks in the previous session, while the mid-cap FTSE 250 was off 0.1%, as of 0716 GMT.

Polling for the parliamentary elections in the UK started at 0600 GMT, and will continue throughout the day with an exit poll scheduled for 2100 GMT.

Opinion polls show that the Conservatives are set to be replaced after 14 years in power by the Labour Party. Investors believe this change is already priced into the market, with the results due early on Friday.

Trading activity in the session is expected to be low as the U.S. markets are closed for a holiday.

Medical equipment and services stocks gained 4.7%, the most among the sectors, as Smith & Nephew jumped 6.7% to the top of the benchmark index after Cevian Capital disclosed an about 5% stake in the company.

The automobile and parts sector climbed 1.2% after an industry body said registration of new cars increased "slightly" in June and crossed the half-year "million motors" mark for the first time since 2019.

Among individual stocks, Barclays gained 1.1% after the bank said it has agreed to sell its German consumer finance business to BAWAG Group AG for a "small premium to net assets", payable in cash. (Reporting by Purvi Agarwal in Bengaluru; Editing by Eileen Soreng)