GlaxoSmithKline has extended its $2.6 billion offer to buy long-time partner Human Genome Sciences until the end of June as it battles the US biotech company's reluctant management.

The price remains unchanged at $13 a share under the longer tender, which will now expire at 5 pm New York time on June 29, Britain's biggest drugmaker said on Friday.

The initial tender period ran out on June 7.

People familiar with the situation previously told Reuters that GSK was set to extend its tender offer - a direct appeal to Human Genome shareholders over the heads of management - while it begins a process to replace the entire HGS board with its own nominees.

The British company has already started reaching out to executives in the drug industry as well as finance and governance experts who could be nominated as independent directors of the 12-member board.

Sources said on May 30 that GSK intended to seek approval from Human Genome shareholders to replace the board under a "consent solicitation" process, which could come in the next few weeks. No details on the process were given on Friday.-Reuters