* League leaps above coalition partner 5-Star
* League could use victory to seek upper hand in cabinet
* Centre-left PD party becomes second largest party (Adds quotes, updated results)
By Crispian Balmer and Angelo Amante
ROME, May 26 (Reuters) - The far-right League became Italy's largest party in Sunday's European parliamentary election, surging past its coalition partner the 5-Star Movement, which saw its own support slump.
The vote looks certain to alter the balance of power within the deeply divided government, giving greater authority to League leader Matteo Salvini, who is pushing for swingeing tax cuts in possible defiance of EU budget rules.
"Thank you Italy. We will use your trust well. The first party in Italy will change Europe," a beaming Salvini said in a video posted on Facebook.
With well over half the ballots counted, state broadcaster RAI forecast that the League would win 33.8% of the vote against 17.7% for 5-Star -- an almost exact inversion of the result of national elections a year ago.
The League took just 6.2% in the last EU ballot in 2014 and has seen its support hurtle higher since then thanks largely to Salvini's uncompromising stance against illegal immigration.
Voting projections said the opposition, pro-Europe Democratic Party (PD) had also leapfrogged above 5-Star, winning 22.5%, compared with 18.7% in 2018 and 40.8% in 2014.
"We are back," said the PD's new leader Nicola Zingaretti.
Relations between the League and 5-Star deteriorated during the election campaign and there has been speculation that the coalition could collapse following the vote because of big differences over issues such as taxes and regional autonomy.
Salvini has said repeatedly the election would have no bearing on the make-up of the government and has denied suggestions he would demand more ministerial positions for his party in the event of victory.
However, he has also said he expects 5-Star leaders to drop their resistance to projects close to his party.
These priorities include his drive to introduce a flat tax in the 2020 budget, regardless of fears this would push the deficit beyond EU limits. He has also called on 5-Star to drop objections to major infrastructure programmes, including a costly trans-Alpine rail link known as the TAV.
"Deciding the priorities of the government will now be up to Matteo Salvini and the League," said the League's parliamentary leader, Riccardo Molinari.
5-Star has traditionally fared better at general elections, when turnout is much higher, and it looked to have been hit on Sunday by low voter numbers in its southern Italian stronghold.
"We have been penalised by abstentions, especially in the south. But now we must put our heads down and work," 5-Star leader Luigi Di Maio said in a brief comment after the scale of the defeat became clear.
The loss to the PD represented a painful blow to 5-Star and means Di Maio is likely to face pressure from party faithful not to make any major concessions to Salvini, which might further erode grass-root support.
Partial results showed former prime minister Silvio Berlusconi's Forza Italia party winning just 8.9% -- its worst performance since it was formed 25 years ago. The nationalist Brothers of Italy party took 6.4%.
Both parties were long-standing allies of the League at a national level until last year, when Salvini split from them to form a cabinet with 5-Star.
However, they still stand together in local votes and their candidate triumphed in a separate regional election on Sunday in Piedmont -- home to the TAV rail project -- with the centre-left coming second and 5-Star trailing badly in third place. (Reporting by Angelo Amante and Crispian Balmer; Editing by Daniel Wallis)
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