By Abigail Summerville

NEW YORK, April 28 (Reuters) - Shareholder advisory firm Institutional Shareholder Services Inc (ISS) urged investors in TravelCenters of America Inc to vote for the company’s planned $1.3 billion sale to BP Plc , saying it "presents higher certainty of completion."

In March, convenience store operator Arko Corp submitted an unsolicited rival bid worth $1.4 billion, or $92 per share, for TravelCenters. TravelCenters has so far rebuffed Arko's advances, saying the deal had a high level of execution risk due to the latter's inability to secure committed financing.

Service Properties Trust , which owns most of TravelCenters' properties, has also raised concerns about Arko's sub-investment grade credit rating.

In a report on Friday, ISS supported TravelCenters' decision to stick with the proposed deal with oil major BP.

“The downside risk of BP walking away and Arko not being able to complete a deal likely is significant, and TA appears to have raised some legitimate concerns about Arko's financing commitments,” ISS said in the note, which was seen by Reuters.

Arko, TA and BP did not immediately respond to requests for comment.

Shareholders of TravelCenters will vote on May 10 on whether to approve the proposed deal with BP.

In February, BP agreed to buy TravelCenters for $1.3 billion. The BP offer of $86 per share represented an 84% premium to the 30-day average trading price for the period ended Feb. 15.

TravelCenters owns a network of about 281 highway sites across 44 U.S. states and offers services beyond fueling, including truck maintenance, restaurants, travel stores and parking.

TravelCenters shares were down 0.1% at $86.11 on Friday afternoon.

(Reporting by Abigail Summerville in New York Editing by Anirban Sen and Matthew Lewis)

((abigail.summerville@thomsonreuters.com;))