EPA chief Lisa Jackson suddenly resigned last week because she was convinced that President Obama is planning to green-light the controversial Keystone XL oil pipeline, The Post has learned.

“She was going to stay on until November or December,” said a Jackson insider. “But this changed it. She will not be the EPA head when Obama supports it [Keystone] getting built.”

While the State Department — not the Environmental Protection Agency — is responsible for the pipeline process because it’s an international project, Jackson is still the president’s top adviser on ecological policy.

EDITORIAL: SAY GOODNIGHT, LISA

She has expressed concerns over the proposed $7 billion pipeline that would carry Canadian oil sands through the US to refineries in the Gulf of Mexico.

Obama early this year rejected the Keystone project, but the pipeline’s operators have refiled their applications. And Jackson has told insiders that the president will approve the project this time — perhaps as soon as March or April.

Pipeline advocates charge it would be irresponsible to kill Keystone because it would create a windfall of jobs and economic activity.

Critics say the process of extracting oil from the tar sands produces excessive greenhouse-gas emissions.

Jackson’s spokeswoman, Victoria Rivas-Vazquez, referred back to the original announcement that she wanted to “pursue new challenges, time with her family and new opportunities.”

She said “the idea that her decision was made based on anything else is entirely false.”

The White House would not comment on Jackson’s rationale for resigning. Spokesman Clark Stevens said the “State Department’s assessment (of Keystone) is ongoing and any speculation would be premature.”