* Inmarsat rejects 532 p/shr offer
* Echostar seeking talks with the board
* Shares down 10 percent (Writes through with context)
LONDON, July 6 (Reuters) - U.S. satellite group EchoStar urged Britain's Inmarsat to open talks about a takeover approach worth $3.2 billion to shareholders just hours before a deadline for the American company to make a firm offer was due to expire.
Shares in Inmarsat fell 8.5 percent on Friday morning to 481 pence after EchoStar revealed it had made a cash and shares bid of 532 pence per share on July 3 that Inmarsat had rejected the following day.
EchoStar, which is chaired by entrepreneur Charlie Ergen, faces a deadline of 1700 London time (1600 GMT) on Friday to make a firm offer or walk away from Inmarsat for six months, under rules laid down by Britain's Takeover Panel.
The U.S. firm said it would proceed with its 532 pence-a-share bid, which values London-listed Inmarsat's equity at 2.45 billion pounds ($3.2 billion), if Inmarsat secured a deadline extension from the panel.
But the British company said in response that EchoStar's bid "very significantly undervalued Inmarsat and its standalone prospects."
It added: "The board remains highly confident in the independent strategy and prospects of Inmarsat."
Including Inmarsat's convertible bonds, EchoStar said its offer valued the UK group's equity and convertible debt at 3.2 billion pounds.
SECOND TIME AROUND
It marks EchoStar's second bid for Inmarsat, after the British satellite operator revealed last month that it had spurned an approach from the U.S. group, which sent the FTSE 250 company's shares surging. Just over two weeks later France's Eutelsat Communications said it had dropped its interest in the UK company after contemplating a rival approach.
Some analysts have speculated that Inmarsat is attractive to EchoStar because of its spectrum in the U.S..
RBC Capital Markets analysts said last month that Inmarsat's spectrum looked particularly complementary to the frequencies owned by EchoStar's sister company Dish Network Corp, the U.S. satellite television group also chaired by Ergen.
Founded in 1979, London-based Inmarsat was set up by the International Maritime Organization as a way for ships to stay in communication with shore and make emergency calls.
Since then, the group has become a private company, providing communications for the British armed forces, aircraft, ships, broadcasters, aid agencies and governments through its fleet of 13 satellites.
Colorado-based Echostar supports similar organisations including the U.S. government with its 23 satellites.
If the American firm succeeds in agreeing a deal with Inmarsat, any takeover is expected to be closely scrutinised by the British government because of the UK company's position as a strategic asset. ($1 = 0.7554 pounds) (Reporting by Kate Holton and Ben Martin Editing by Keith Weir)