(Adds rating context and Moody's background)

April 25 (Reuters) - Boeing's credit rating was under pressure for the second time this week as S&P brought them one step closer to a junk rating on Thursday after a mid-air blowout of a cabin panel in January prompted the planemaker to slow production of its best-selling jets.

The rating agency changed its outlook on the company's bottom of investment grade rating to "negative" from "stable" to reflect an increased potential for Boeing to face further delays in expected recovery of its cash flow and credit ratios.

Moody's cut Boeing's credit rating to one notch above junk status on Wednesday just hours after the company reported it used $3.93 billion of free cash flow in the first quarter.

The planemaker has been under heavy regulatory scrutiny after the cabin panel blew off an Alaska Airlines flight in mid-air, forcing an emergency landing.

Boeing faces heightened production uncertainty, notably related to quality issues affecting its 737 MAX aircraft, and key changes to its leadership are pending, S&P said on Thursday.

On Wednesday, Moody's said it expects headwinds surrounding the company's commercial airplanes to persist at least through 2026 when Boeing has $8 billion in debt coming due.

S&P expects Boeing will not generate sufficient free cash flow in 2024 to cover about $5 billion of its debt maturities.

The rating agency also flagged concerns around leadership uncertainty at the planemaker, leading to additional pressure to its rating.

S&P affirmed Boeing's "BBB-" long- and "A-3" short-term issuer credit ratings. (Reporting by Shivansh Tiwary in Bengaluru; Editing by Shounak Dasgupta and Chizu Nomiyama)