LONDON — The investment company Atlas Mara Co-Nvest, backed by Robert E. Diamond Jr., the former Barclays chief executive, said on Friday that it had increased its stake in Union Bank of Nigeria in its latest bet on the African financial sector.
The company said that it had exercised an option to acquire 20.9 percent of Union Bank from Asset Management Corporation of Nigeria for about $270 million.
Following the transaction, Atlas Mara will own 29.9 percent of the Nigerian bank. The company had previously owned shares through its subsidiary, ADC African Development Corporation.
Founded in 1917, Union Bank offers financial services to retail and business customers, primarily in Nigeria. As of June 30, it had about $6.3 billion in assets and about 340 branches throughout Nigeria.
“This is a very significant acquisition for Atlas Mara,” Mr. Diamond said in a news release. He added that the deal would give Atlas Mara exposure to what it considered strategic markets in southern Africa, East Africa and West Africa.
The transaction is subject to regulatory approval.
Union Bank was established under the name Colonial Bank in 1917 and was acquired by Barclays in 1925. It operated under the name Barclays Bank DCO until the 1970s when it went through structural and ownership changes.
Atlas Mara was formed last year by Mr. Diamond and Ashish J. Thakkar, an entrepreneur whose Mara Group conglomerate has technology, manufacturing and real estate interests in 19 African countries.
The company relisted on the London Stock Exchange in August after it completed its acquisitions of ABC Holdings, which operates the African bank BancABC, and of ADC.
Atlas Mara also agreed in May to acquire a majority stake in the commercial arm of the Development Bank of Rwanda.
The company raised about $325 million last year. It had aimed for an additional $400 million in its fund-raising earlier this year, but came up short. It eventually raised about $300 million.
“We are well on the way to becoming a leading African financial institution that will benefit from, and contribute to, the continued growth of rapidly expanding sub-Saharan Africa,” Mr. Thakkar said in a news release.
Shares of Atlas Mara were down slightly to $10.20 in trading in London on Friday morning.