Under the Basel III accords, minority stakes in a bank's units held by third parties will be valued less highly in terms of their contribution to that bank's core capital - giving the bank an incentive to buy them back.
Banks across Europe have been beefing up their capital ratios ahead of region-wide financial health checks designed to identify and weed out lenders at risk of going bust in the event of another financial crisis.
UBI (>> UBI Banca Scpa) said in a statement it would pay 327 million euros (269 million pounds) to repurchase the minority stakes held by Aviva Holding Italia in Banca Popolare Commercio e Industria, Popolare di Ancona and Banca Carime.
UBI estimated the payment would take about 35 basis points off its Core Tier 1 capital ratio as of end-December. This measure of best-quality bank capital stood at 12.6 percent of UBI's risk-weighted assets at end-2013.
UBI shares rose sharply and traded up 5.25 percent at 6.815 euros by 0932 GMT, outpacing a 2.6 percent rise <.FTIT8300> in Italy's banking index.
Traders said the gains were triggered by a bullish report by Mediobanca Securities, published before the repurchase transaction was announced but which said the bank should continue to rationalise its minorities.
"We see UBI's capital ratios as appropriate. Nevertheless and regardless of the incoming asset-quality review, UBI should keep pursuing the rationalization of minorities which could add up to 75 basis points" to the bank's core capital under Basel III, the brokerage said.
It raised its price target on UBI to 8.1 euros and held its "outperform" rating on the stock.
UBI said it would at the same time reap a 57 million euro net gain from selling Aviva a further 30 percent stake in two life bancassurance ventures that were previously jointly controlled.
A banking analyst who declined to be named said the overall impact of the renegotiated accord on UBI's capital was broadly neutral.
Aviva (>> Aviva plc) and UBI extended existing commercial distribution accords, due to expire in 2015, until the end of 2020.
Aviva shares rose 1.5 percent.
(Refiles to add dropped word 'units' in second paragraph)
(Reporting by Valentina Za; Editing by John Stonestreet)