ISTANBUL—Turkish authorities seized Bank Asya, ending a long-running battle to bring the Islamic lender firmly under government control after the company became embroiled in a political feud.

The move, announced by Turkey's Banking Regulation and Supervision Agency late Friday, comes just a week before critical parliamentary elections and marks the latest step in an 18-month saga that saw the country's largest Islamic lender lose half of its assets as political attacks fueled a run by depositors.

Bank Asya now belongs to the state-run Savings Deposit Insurance Fund, which answers directly to the prime minister.

"It has been determined that problems experienced in the bank's financial structure, partnership and management makeup, as well as issues in its operations, present a threat the confidence and stability of the financial system," the banking watchdog said in a statement, adding that investors' rights were also at risk.

Bank Asya's total assets amount to less than 1% of Turkey's $800 billion banking industry. In February, the banking watchdog transferred 63% of the lender's preferred shares to the Savings Deposit Insurance Fund, giving it control of Bank Asya's management and stoking concern among investors that the government was deploying regulators to settle political scores.

The bank said Friday's regulatory takeover wouldn't affect its operations, and that the lender "will continue to provide quality services." The state-run fund has been given the task of arranging a partial or wholesale transfer of Bank Asya to another firm, including through a sale or merger, the banking watchdog said.

Officially known as Asya Katilim Bankasi AS, the lender emerged as a major front in a bitter struggle between President Recep Tayyip Erdogan and a U.S.-based Turkish cleric, Fethullah Gulen, whose followers founded the bank.

Bank Asya had been a star in Turkey's effort to boost the market share of Islamic banks, which operate under the principles of Islamic law, or Shariah, sharing profits and losses, while prohibiting the collection and payment of interest. Foreign investors including BlackRock Inc. and Vanguard Group had held about a third of the 54% of shares that were publicly listed.

But the bank's fortunes turned after prosecutors associated with Mr. Gulen's religious network pressed corruption charges against Mr. Erdogan's ministers and business allies in December 2013. Mr. Erdogan, then prime minister, accused his one-time ally of plotting a coup to topple him, while pro-government newspapers attacked Bank Asya as the financing arm of the imam's push to take over the state.

Mr. Gulen, who lives in self-imposed exile in Pennsylvania, and Bank Asya deny the charges. The cases against Mr. Erdogan's allies were dropped by prosecutors who replaced the original attorneys on the sprawling corruption investigation.

As Mr. Erdogan started going after Mr. Gulen's followers in the bureaucracy, removing from their posts thousands of police officers, prosecutors and judges suspected of sympathizing with the imam, regulators started reviewing the bank. State-run firms and government clients withdrew deposits, and Mr. Erdogan said the lender was bankrupt, a claim the bank disputed.

The president also scuttled partnership talks with Qatar Islamic Bank, while one of his advisers derailed a potential Bank Asya takeover by T.C. Ziraat Bankasi AS, the state-owned lender that is Turkey's biggest bank by assets.

"Turkey's business environment is becoming increasingly politicized," said Anthony Skinner, a political risk consultant covering Turkey at U.K.-based firm Verisk Maplecroft. "Investors have long complained about changing regulations and arbitrary decision-making—the politicized nature of decision-making has become all the more apparent over recent years."

The government is also targeting companies outside of Mr. Gulen's network, Mr. Skinner said, citing Turkiye Is Bankasi AS and Koc Holding AS, the biggest Turkish conglomerate, which faced tax probes after Mr. Erdogan accused the firm of supporting antigovernment protests.

Officials deny allegations that Bank Asya was seized because of political reasons. Deputy Prime Minister Ali Babacan, who oversees the economy, on Saturday said regulators wouldn't have waited for more than year if the decision was politically motivated, adding that he "presumes" the banking watchdog has strong legal grounds.

Mr. Erdogan and the government have made eradicating what they call Mr. Gulen's "parallel state" from Turkey a key campaign issue in last year's local and presidential ballots, as well in as the parliamentary elections that will take place next Sunday.

While most investors compartmentalize politicized regulatory actions against Bank Asya as a unique incident in a broader battle, they also point to it as the beginning of a potentially worrisome trend. Fitch Ratings has called the episode a "black mark" on Turkey's prudent banking regulations since the 2001 financial crisis.

But criticism hasn't deterred Turkey's leadership.

"Those who betray this country will pay for it," Mr. Erdogan said last week in a speech to small-business owners, asking them to support the government in the elections. "We need unity."

Write to Emre Peker at emre.peker@wsj.com

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