* English soccer rights auction begins on Friday
* Amazon has made aggressive move into TV
* BT and Sky not expected to pay for big increases
* Future of Sky uncertain due to Murdoch takeover bid
By Kate Holton and Paul Sandle
LONDON, Feb 5 (Reuters) - Amazon's sports broadcasting ambitions will be tested this week in a multi-billion pound auction of English Premier League soccer rights, potentially pitting it against Sky and BT.
England's auction for the rights to screen matches including Manchester United and City, Liverpool and Chelsea is one of the biggest money spinners in world sport, with the last three-year domestic package raising 5.14 billion pounds ($7.25 billion).
Broadcasters who have stumped up for the best packages to win viewers and fend off rivals could now face another threat from one of the big U.S. tech groups entering the fray.
Amazon, the world's largest retailer, has moved aggressively into TV to bolster its Amazon Prime membership service, which offers free delivery and content for a flat monthly fee, and the new auction appears to have been structured specifically to attract a digital player for at least a small set of games.
That is likely to force Rupert Murdoch's Sky and Britain's biggest telecoms group BT to increase their offers, but financial and strategic pressures mean analysts do not think they will match previous 70 percent jumps.
"(There is a) very real threat that Amazon will look to take at least some of the UK and later on, international rights," Guy Bisson from the media analyst firm Ampere said ahead of the auction which begins on Friday.
Amazon, which in 2015 tapped up the Top Gear presenters Jeremy Clarkson, Richard Hammond and James May to produce a new series called the Grand Tour, has also won the rights to some of America's National Football League and ATP tennis.
Sky refused to discuss its commercial strategy, while Amazon declined to comment.
The English auction for the three seasons beginning 2019/20 will make 200 live games available out of the 380 played each season, divided into seven lots, with five packages consisting of 32 games and two packages of 20 games.
One package will include the rights to show a whole round of matches at the same time, an option that could be more attractive to a digital provider than a traditional broadcaster.
In the previous auction Sky, which built its business on the back of the Premier League, picked up 126 games to BT's 42 and analysts expect they will want to achieve at least a similar outcome this time around.
Both are to some extent limited, however.
Sky, present in 13 million homes, had to cut costs, hike prices and drop other sports to afford the last round of rights.
It also has an uncertain future as it is not clear who will own Sky when the 2019 season begins, with Murdoch's 21st Century Fox trying to buy the 61 percent it does not own. Sky could then be sold to Disney if a separate sale of Murdoch's TV and film assets receives the green light.
For BT, investors would be unlikely to welcome a blow-out bid with its shares at five-year lows. It faces other calls on its cash, including investments in ultrafast fibre, pension deficit top-ups and dividend payments.
But the companies have recently agreed a wholesale deal to allow their customers to watch the other service's channels, in an easing of the previous competition in their relationship.
"We continue to see the Premier League content as being an important part of BT Sport but it's only one part of the channel," BT Chief Executive Gavin Patterson said last week.
"We know what it's worth to us, and we model that and we bid up to but no further than the value of it, and we always have a plan B if we do not get the content we want." ($1 = 0.7093 pounds) (Reporting by Kate Holton; editing by Alexander Smith)