By Jeannette Neumann and Patricia Kowsmann 

Caixabank SA on Tuesday launched 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 become the dominant bank across the Iberian Peninsula.

Caixabank, Spain's third-largest lender by market value, said it would pay EUR1.329 a share in cash, representing a 27% 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 in terms of assets and loans. Fifty-percent of the BPI shareholders must approve the takeover bid, Caixabank said. The bank expects the deal to close by the end of the second quarter.

Caixabank shares fell 3% to close at EUR3.98, while shares of BPI rose 27% to close at EUR1.33.

Caixabank's offer comes as BPI considers its own bid for Novo Banco SA, the lender that was carved out from the collapse of Portugal's Banco EspíSHYrito Santo SA.

If Novo Banco is purchased by BPI and BPI is in turn swallowed up by Caixabank, that would transform Caixabank into the largest lender in Portugal, with a roughly 28% market share of assets and loans, J.P. Morgan & Chase Co. analysts said in a research note Tuesday.

Such a foothold in Portugal would convert Caixabank into the dominant lender 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 and other markets such as the U.K. and U.S., Caixabank has the greatest number of bank branches and the largest market share within Spain.

Caixabank Chief Executive Gonzalo Gortázar told analysts during a presentation Tuesday that the takeover bid for BPI makes sense regardless of whether the Portuguese lender later makes a move to purchase Novo Banco. "We are presenting this proposal today because it makes sense on its own," he said. "It's the right move for Caixabank. It's the right move for BPI. And that's true irrespective of the possibility of Novo Banco being sold."

Buyers interested in Novo Banco haven't submitted formal offers yet. The Bank of Portugal is hoping for a purchase price as close as possible to the EUR4.9 billion that had to be injected into the bank through a resolution fund.

Another condition of the takeover, Caixabank said Tuesday, is that BPI shareholders must vote to remove a rule that restricts Caixabank to voting rights equivalent to 20% of BPI's capital even though the Spanish bank owns 44.1% of the Portuguese lender's capital.

"Our voting and economic rights should be aligned," Mr. Gortázar said. For the voting-rights cap to be lifted, 75% of BPI shareholders have to approve its removal.

One of those shareholders is Africa's wealthiest woman, Isabel dos Santos. Ms. dos Santos is the daughter of Angola's president and is the second-largest shareholder in BPI after Caixabank. A company owned by Ms. dos Santos holds 18.6% of BPI's shares and "holds the key for the Caixabank bid as it has to support the lifting of the voting rights limit," Francisco Riquel, an analyst with Madrid-based financial-services firm N+1 Group, said in a research note.

BPI controls 50% of Angolan bank Banco de Fomento Angola SA. If Caixabank's bid for BPI is successful, Mr. Riquel said, the bank is likely to sell the Portuguese lender's stake in the Angolan bank to the Angolan government, "leaving BPI as a pure player in Portugal."

Caixabank said in a regulatory filing Tuesday morning that a purchase of BPI 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.

Assuming 75% of shareholders approve the deal, that would trigger a 1.1-percentage-point decrease in its capital ratio to 10.4% based on December 2014 figures, Caixabank executives said Tuesday.

The bank said it plans to have a fully loaded capital ratio, which are the requirements under the latest regulatory guidelines, above 11% at the end of 2015.

"We are slightly concerned about Caixa's capital levels falling to 10.4% versus (an) 11% target," Citigroup analyst Stefan Nedialkov wrote in a research note Tuesday.

Mr. Nedialkov said Caixabank might raise capital to plug the gap. Caixabank executives didn't rule out that possibility.

Like other Portuguese banks, BPI's profitability has been hurt by a severe downturn in the country. In 2011, the government was forced to request a bailout from European Union peers and the International Monetary Fund.

The recession led to a rise in souring loans at lenders. BPI, however, has been among the most resilient. Portugal's economy is now recovering, and analysts say BPI should be profitable again this year. It posted a EUR47.3 million net loss in the fourth quarter.

"We are bullish on the medium- and long-term prospects of Portugal, even though there is a lot of work to be done, like in Spain," Mr. Gortázar said.

Write to Jeannette Neumann at jeannette.neumann@wsj.com and Patricia Kowsmann at patricia.kowsmann@wsj.com

Access Investor Kit for Banco Bilbao Vizcaya Argentaria SA

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=ES0113211835

Access Investor Kit for Banco Santander SA

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=ES0113900J37

Access Investor Kit for CaixaBank SA

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=ES0140609019

Access Investor Kit for Banco BPI SA

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=PTBPI0AM0004

Access Investor Kit for Banco Bilbao Vizcaya Argentaria SA

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=US05946K1016

Access Investor Kit for Banco Santander SA

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=US05964H1059

Access Investor Kit for CaixaBank SA

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=US12803K1097

Access Investor Kit for JPMorgan Chase & Co.

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=US46625H1005

Subscribe to WSJ: http://online.wsj.com?mod=djnwires