WASHINGTON (Reuters)—Several U.S. lawmakers plan to recommend the shutdown of the BP-operated Atlantis oil and gas platform in the U.S. Gulf of Mexico until federal regulators can prove the region's second-biggest rig is operating safely.

A group of lawmakers will urge an expanded investigation by the Minerals Management Service into Atlantis, which pumps up to 200,000 barrels per day of crude, according to a draft of the letter they will deliver to Interior Secretary Ken Salazar on Wednesday, obtained by Reuters.

"We urge MMS to listen to the expert engineer who reviewed the Atlantis situation and called for an immediate shut-down until it can be shown that this platform is operating safely," the letter said. It was not immediately clear how many U.S. lawmakers would sign the letter on Wednesday, according to a source familiar with the efforts.

A Feb. 24 letter from lawmakers to the MMS urging an Atlantis safety investigation was signed by 19 lawmakers, and led by Arizona Democrat Rep. Raul Grijalva, D-Ariz., who sits on the Congressional Committee on Natural Resources and chairs its subcommittee on National Parks, Forests and Public Lands.

Atlantis, one of the most complex deepwater platforms in the world, pumps crude and natural gas from the Green Canyon blocks in the U.S. Gulf and is located in waters more than 7,000 feet deep, around 150 miles south of New Orleans. BP P.L.C. is the operator at Atlantis and a 56% stakeholder, while BHP Billiton holds a 44% working stake.

Interior Secretary Ken Salazar said in Senate testimony on Tuesday the U.S. government was investigating Atlantis and admitted his agency came up short in preventing the recent Gulf of Mexico oil spill.

Any potential shutdown at Atlantis, even short-lived, could knock out a major chunk of BP's U.S. crude and natural gas production. The platform can produce up to 200,000 barrels a day of crude, around 13% of total U.S. Gulf output, and up to 180 million cubic feet of natural gas per day.

BP, the top hydrocarbons producer in the U.S. Gulf, has U.S. production of more than 400,000 barrels per day of oil equivalent and operates the two largest platforms in the Gulf, Thunder Horse and Atlantis.

Regulators are increasing scrutiny of offshore drilling rigs in the U.S. Gulf following the deadly rig explosion and ongoing oil spill from BP's Macondo field in the Gulf. So far, the Horizon oil spill has yet to significantly affect U.S. oil production, but any order to halt Atlantis could change that.

Lawmakers expect MMS to complete its investigation into Atlantis by the end of May. The MMS did not immediately return phone calls seeking comment.

In the letter, lawmakers voiced new concerns that the Atlantis oil and gas project, which began production in 2007, has operated without up-to-date "as built" engineering documents and diagrams showing how all of its components work.

The poor documentation could lead to an accident at Atlantis with the potential to be even more damaging than the Horizon spill, the draft letter warns.

"We are very concerned that the tragedy at Deepwater Horizon could foreshadow an accident at BP Atlantis, which is operating in deeper water than Horizon," according to one passage. The "worst-case scenario spill" could be many times worse than the Horizon disaster, exceeding the volume of the Exxon Valdez spill of 1989 in just two days time, the letter says.

BP spokesman Toby Odone said on Wednesday that the company wouldn't comment on the letter, since BP hasn't seen it.

On Monday, BP said Atlantis has been operating to rigid safety standards. The company rebuffed claims from a former BP contractor, Kenneth Abbott, who has said BP operated the rig without complete or accurate engineering documents.

Mr. Abbott, along with advocacy group Food and Water Watch, filed a federal lawsuit this week against Mr. Salazar and the MMS, seeking to have the Atlantis shut down pending safety investigations.

BP's earlier investigation into the matter found that Mr. Abbott's claims were "without substance," the company said.

Rep. Grijalva and other lawmakers will urge the MMS to interview Mr. Abbott and other experts as part of its Atlantis investigation. According to a database Mr. Abbott compiled while on contract with BP, more than 90% of the engineering documents and drawings for Atlantis' subsea components had not been approved by an engineer as required by regulations.

The lawmakers said in their letter that an internal BP email from 2008 indicated that the company recognized incomplete or inaccurate engineering documents on Atlantis "could lead to catastrophic Operator errors."