Royalty Pharma said today that it was standing by its offer to acquire Elan, which has rejected the approach.

The US investment company said it was ready to move quickly and could complete due diligence in 20 days.

In a statement, Royalty also said it was meeting with Elan shareholders.

"Royalty Pharma believes Elan shareholders should urge the board of Elan to engage with Royalty Pharma and grant the required access to allow an offer to be pursued expeditiously," the company said today.

Royalty Pharma made a $6.6 billion approach last week after Elan announced last month a $3.2 billion plan to sell its 50% stake in multiple sclerosis drug Tysabri to its US partner, Biogen Idec.

Elan rejected Royalty Pharma and on Monday sweetened its offer to its shareholders, saying it would give them 20% of future royalties from Tysabri. It had already planned to return $1 billion to shareholders after the drug sale.

"The 'proposal' by Royalty Pharma remains an indication of interest, is highly conditional, and may or may not lead to an offer being made for the entire issued share capital of the company," Elan said today.

"The highly conditional indication of interest is opportunistic in its timing,'' the Dublin-based company added.

In today's statement, Royalty criticised Elan for not taking its bid to Elan shareholders. It also criticised Elan's sweetened offer to shareholders.

"Royalty Pharma believes that Elan is still not focused on maximizing shareholder value, and that Elan shareholders should treat Elan's recent change of heart regarding returning cash to Elan shareholders with skepticism," it said.

Royalty Pharma questioned whether Elan would have any value at all after the Biogen deal is completed. It also said that as the world's largest buyer of pharmaceutical royalty revenue streams, Royalty Pharma may be the only buyer for Elan.

Earlier this week, Elan's chief executive Kelly Martin said his company did not view Royalty Pharma's offer as credible and said the majority of its investors did not believe the bid was worthy of a discussion period.

He also said he expects the Tysabri deal to close in a month or two.