The Massachusetts Bay Transportation Authority's operating budget is unsustainable, with expenses increasing at nearly three times the rate of revenue growth, according to a report by the authority's new fiscal oversight body.

Left unchecked, the structural operating deficit of the MBTA will reach $427 million in fiscal 2020, the Fiscal Oversight and Control Board said in its initial report Tuesday.

The MBTA operates Greater Boston's mass transit system.

In addition, annual capital spending on deferred maintenance and capital investment - the so-called state-of-good-repair backlog -has historically fallen substantially below the $472 million annual spending needed to prevent the maintenance backlog from worsening. It averaged $378 million a year from fiscal 2009 through 2014.

These problems at are even more serious than what a special panel reported to Gov. Charlie Baker in May, according to the report.

"The initial findings are sobering," said the report. "The MBTA's structural budget is broken, with the rate of growth in operating expenses and debt service continuing to exceed revenue growth."

The panel began two months ago. Lawmakers, at Baker's behest, formed it after a record 110 inches of snow in the region crippled several subway and commuter lines.

The board's analysis of MBTA finances, operations, and management identified significant structural challenges, notably rising expenses. It also calculates the state-of-good-repair backlog for the first time.

State of good repair is transportation parlance for basic maintenance.

The scathing report came one day after Standard & Poor's downgraded the MBTA's $3.7 billion of parity sales tax bonds outstanding to AA-plus from AAA. MBTA's capital needs could result in "substantial" additional bonding," S&P's David Hitchcock said in a statement.

S&P assigned its AA-plus long-term rating to the MBTA's Series 2015A senior sales tax bonds. Both sets of bonds carry stable outlooks.

"Without question, this [control board] report paints a bleak picture of the current state of the MBTA," said control board chairman Joseph Aiello, who added that the findings provide a necessary baseline for correcting the system.

The control board said the maintenance backlog has surged to $7.3 billion, reflecting prolonged underspending and improvements to the state-of-good-repair database.

Board members are working with new MBTA management -- Frank DePaola succeed Beverly Scott as general manager in the spring -- to build an operating budget model for fiscal 2017 through 2020 that will feature cost containment and increased own-source revenue, or revenue generated from such sources as parking, advertising and real estate public-private partnerships, also called "value capture."

Other reports will follow, including the control board's initial annual report on Dec. 15.

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