(Adds details of other stakeholders in paragraph 2, Galp share movement in paragraph 5, Bank of America decline to comment in paragraph 10)

LONDON, April 22 (Reuters) - Portuguese oil company Galp Energia has launched the sale of half of its stake in an exploration block offshore Namibia where it has made a major oil discovery, three industry sources familiar with the matter told Reuters.

Galp has an 80% stake in Petroleum Exploration Licence 83 (PEL 83), which covers almost 10,000 square kilometres in the Orange Basin, with Namibia's national oil company NAMCOR and independent exploration group Custos each holding another 10%.

The block includes the Mopane field that Galp estimates holds at least 10 billion barrels of oil and gas equivalent after completing the first phase of exploration.

Galp is seeking to sell a 40% stake in the block, said the sources, who could not be named because they are not authorized to speak to the media.

Shares in the company were up 17% by 0950 GMT.

Galp declined to comment. It has previously indicated it could launch a process to attract other investors to its projects in Namibia, as they could reach a large scale.

The Mopane discovery appears to be one of the largest made in the nascent basin following successful exploration campaigns by rivals TotalEnergies and Shell in recent years. It could help kickstart the southern African country's oil industry.

Namibia has attracted huge interest from international oil companies seeking to grow their production as demand is forecast to remain strong for years even as governments seek to reduce their fossil fuel consumption.

Galp is also offering to cede control of the development of the project to the potential buyer, expected to be a major international rival with a strong track record in project management, the sources said.

It has hired Bank of America to run the sale process, which could raise several billion dollars for Galp, although the exact value is still unclear, the sources said. Bank of America declined to comment.

Namibia could become a new source of revenue for Galp, which currently has strong investments off the coast of Brazil and is also present in a natural gas project in Mozambique's Rovuma basin. (Reporting by Ron Bousso; Editing by Emelia Sithole-Matarise)