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* FTSE 100 up 0.3 pct
* Inmarsat shoots up as SoftBank speculation swirls
* 3I tops FTSE after Barclays upgrade
* Polls send sterling lower, boosting large-caps
LONDON, June 1 (Reuters) - Britain's major share index climbed on Thursday, flirting with its record high level as a weaker sterling gave large multinationals a leg up, while Inmarsat rose on merger speculation.
The FTSE 100 was up 0.4 percent by 0900 GMT, with consumer staples and industrials stocks providing the top boosts to send it hovering near its highest intra-day level of 7,586.45 points hit on Wednesday.
London-listed multi-national firms, which dominate the index, benefit when sterling weakens. Pressure on the currency has been increasing over the past week as some opinion polls point to a tighter-than-expected race in next week's election.
The latest poll published by the Times late on Wednesday had May leading by just 3 points ahead of the opposition Labour party.
Mid-caps lagged the larger stocks slightly, up 0.2 percent.
"We have seen a relative underperformance of domestic focused UK equities as political risk increases alongside the perceived uncertainty of the Brexit outcome," said Edward Park, investment director at Brooks Macdonald.
But Inmarsat gained 5.4 percent on the day, leading the way among European satellite companies, with France's SES and Eutelsat also fueled by speculation they could be takeover targets after sources said Softbank would let its planned $14 billion merger between OneWeb and Intelsat collapse.
While the large-caps were ahead on the day, UBS Wealth Management warned the boost to the FTSE 100 from the weak pound could be turning stale.
"The UK equity market's tailwind from the weak pound is fading, and as we lap the currency low point during the second and third quarters of this year, the market will no longer receive a boost from currency effects," said deputy head of the UK investment office Caroline Simmons.
Gains among large-caps were broad-based and broker calls helped some specific stocks stand out.
Private equity firm 3I Group led large-cap gains, up 3.2 percent after Barcalys raised its price target on the stock, saying a trading update from Action management, which accounts for 30 percent of 3I's portfolio, was reassuring and the group was confident on cash generation.
Mediclinic however sank 3.3 percent, the top FTSE faller, after both Credit Suisse and Bank of America Merrill Lynch cut their rating on the private health-care provider.
Meanwhile bus and rail company FirstGroup fell 5.7 percent after a 23 percent profit jump was overshadowed by its warning of a mixed trading outlook.
Challenger bank Aldermore fell 3 percent after Exane cut it to "underperform," citing slowing loan growth and an uptick in impairments.
Business support services firm G4S and real estate investment trust Segro were confirmed as the latest additions to the blue-chip index, with changes effective on June 16.
Intu Properties and Hikma Pharmaceuticals were set to be demoted from the large-caps to the mid-cap index, as part of a quarterly review reshuffling stocks based on their relative size. (Reporting by Helen Reid; Editing by Alison Williams)