Drugmaker GlaxoSmithKline P.L.C. said Wednesday it would play a bit more hardball with Human Genome Sciences Inc., taking its $2.6 billion takeover offer directly to shareholders after HGS management declined a friendlier overture on April 11.

Glaxo is based in London but has a big operation in Center City and facilities in the Philadelphia suburbs. HGS is based in Rockville, Md.

On April 18, HGS disclosed Glaxo's offer of $13 a share. HGS management said in a statement then that the offer undervalued the company and invited Glaxo to participate in a "strategic review process."

Both sides seem to agree that such a process would only take time and raise the price, so Glaxo declined, though it said again Wednesday that it would happily continue discussions with current HGS management. HGS shares closed down 3 cents, to $14.59.

Glaxo said that its original offer was plenty valuable — an 81 percent premium on the April 18 stock price of $7.17 — and that it would go directly to shareholders with that offer.

"There is clear strategic and financial logic to this combination, and HGS shareholders should have the opportunity to decide for themselves on the merits of the offer," Glaxo said in a statement.

Part of Glaxo's confidence stems from partnerships it already has with HGS on several of HGS's key products, including the lupus drug Benlysta. That might deter other bidders for HGS, because they would be buying only a piece of those assets.

HGS said in a statement Wednesday that its board of directors, in consultation with financial and legal advisers, would carefully review the offer. The company said it would file a recommendation for shareholders with the Securities and Exchange Commission within 10 business days of Glaxo's starting the unsolicited bid, and advised shareholders to take no action in the meantime.

Contact David Sell at 215-854-4506, dsell@phillynews.com or follow on Twitter @PhillyPharma. Read his PhillyPharma blog on philly.com.