By Ben Hirschler

LONDON (Reuters) - U.S. biotech company Human Genome Sciences Inc, fighting a hostile bid from GlaxoSmithKline Plc, set a July 16 deadline for definitive takeover offers on Friday and made another plea for GSK to join the auction process.

GSK, Human Genome's long-time partner, has taken its $2.6 billion bid for the pioneer of gene-based medicine discovery direct to shareholders through a tender offer after Human Genome spurned the $13-a-share offer as inadequate.

Britain's biggest drugmaker - the only company to have made an offer - once again rejected the idea of participating in the auction being run by Human Genome's advisers Goldman Sachs and Credit Suisse.

Human Genome said it had sent a letter to GSK repeating its invitation to join in, but GSK, which argues its offer is full and fair, promptly wrote back turning it down.

"GSK continues to believe that participation in the process is unnecessary, given the company's offer is not conditioned on due diligence or financing and can be completed expeditiously," it said.

The group added it remained willing to meet with Human Genome to discuss the offer at any time.

GSK last week extended its tender offer to June 29, after securing less than 1 percent of the biotech firm's stock in an initial tender round.

People familiar with the situation said GSK had also started a process to replace the entire Human Genome board with its own nominees.

GSK intends to seek approval from Human Genome shareholders to replace the 12-member board under a "consent solicitation" process, which it can launch while its tender is open. GSK also has the option to extend the tender again beyond June 29.

Human Genome, which has also adopted a "poison pill" shareholder rights plan in a bid to thwart the hostile takeover attempt, says it has had contacts with other companies. But no counterbidder has emerged and bankers say GSK has an 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.

Human Genome shares continue to trade just above GSK's $13-a-share offer, adding 1.2 percent to $13.25 by 1430 GMT. That indicates investors still hope for a higher price, although the stock has fallen back from a high of $15.49 hit in April, soon after GSK's offer was made public.

(Editing by Hans-Juergen Peters and David Holmes)