GlaxoSmithKline has once again extended its $2.6 billion offer to buy long-time partner Human Genome Sciences until July 20 as a hostile stand-off with the U.S. biotech company continues.

The offer price remains unchanged at $13 a share.

The decision to push back the closing date for the bid comes as no great surprise, since Human Genome effectively extended the war two weeks ago by setting a July 16 deadline for definitive takeover offers.

"Extension of the tender offer to 20 July will provide HGS shareholders the opportunity to evaluate the outcome of the HGS board's process relative to GSK's offer," GSK said in a statement on Friday.

"Based on circumstances at that time, GSK will consider all available options regarding its offer but can make no assurance that the tender offer will be further extended."

Britain's biggest drugmaker - the only company to have made an offer - has consistently refused to participate in the auction process being run by Human Genome's advisers Goldman Sachs and Credit Suisse.

Human Genome says it has had contacts with other companies, but bankers say GSK has a big advantage over rivals because of its partnerships around key drugs.

The two companies together sell Benlysta, a new drug for the autoimmune condition lupus, and they also collaborate on two other experimental drugs for diabetes and heart disease that could become significant sellers.

GSK and Human Genome share rights to Benlysta, while GSK owns the majority of the commercial upside to the other products.

GSK's tender had been due to expire at 5 pm New York time (2100 GMT) on June 29. It was extended a first time on June 8 after GSK secured less than 1% of the biotech firm's stock in an initial tender round.