* 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.
LEAGUE PRIORITIES
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.