By Jeannette Neumann 

MADRID-- Caixabank SA on Tuesday said it was launching a takeover bid for the 55.9% of Portuguese lender Banco BPI SA that it doesn't already own, as the Spanish lender seeks to bolster its presence in the Iberian peninsula.

Caixabank, Spain's third-largest lender by market value, said it would pay EUR1.329 a share in cash, representing a 28% premium to BPI's closing share price Monday. The price means Caixabank could spend as much as EUR1.09 billion ($1.24 billion) to purchase Portugal's No. 4 bank.

The Spanish bank said 50% of the Portuguese lender's shareholders must approve the takeover bid. BPI's shareholders must also vote to remove a rule that restricts Caixabank to voting rights equivalent to 20% of BPI's capital.

BPI has expressed interest in bidding for Novo Banco, the lender that was carved out from the collapse Portugal's Banco Espírito Santo.

If Caixabank closes the purchase of BPI, and then bought Novo Banco, it would solidify its role as the Spanish bank with the greatest presence in Portugal and across the Iberian peninsula. While Spanish rivals Banco Santander SA and Banco Bilbao Vizcaya Argentaria SA are larger than Caixabank in terms of market value, partly because of their presence in Latin America, Caixabank has the greatest number of bank branches and the largest market share within Spain.

Caixabank said in a regulatory filing Tuesday morning that the purchase would chip away at its "fully loaded" capital ratio by 0.8 to 1.4 percentage points, depending on the level of shareholder acceptance of the deal.

The bank said it plans to have a fully loaded capital ratio, which are the requirements under the latest regulatory guidelines, above 11% after the deal is complete.

Caixabank is hosting an investor call to discuss the takeover bid at 10 a.m. Madrid time.

Write to Jeannette Neumann at jeannette.neumann@wsj.com

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