(Reuters) - Prominent proxy advisory firm Institutional Shareholder Services on Friday recommended Frontier Communications shareholders abstain from voting on Verizon's $20 billion takeover bid and instead support the adjournment proposal.

Shareholders must decide whether a sale to Verizon, which likely would not close for at least another year, represents a better risk-adjusted alternative to the company's standalone option, the proxy advisory firm said in a report.

"Given the possibility of substantially more value down the line, and the lack of urgency to approve a transaction ... it seems reasonable for shareholders to exercise the optionality of abstaining for the time being," ISS said.

Verizon said in September it would buy Frontier at an offer price of $38.50 per share, as the U.S. telecom giant looks to build its fiber subscriber base and better compete with rivals such as AT&T.

The takeover, however, has faced opposition from some Frontier shareholders. Reuters first reported last month that some of the fiber-optic internet provider's largest shareholders were concerned about the deal.

Glendon Capital Management, which owns nearly 10% of Frontier, said last month it had decided to vote against the deal at the current price, as it "significantly undervalues" Frontier's assets and earnings trajectory.

ISS said the firm finds it unlikely that Verizon would "dramatically raise its bid for Frontier at this stage."

Last week, Frontier urged its shareholders to vote for the proposal, saying that it had reviewed interest from other potential buyers and found Verizon's proposal as the most favorable option.

The offer was "highly attractive" and creates significant value for stockholders, it had then said.