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The speaker himself is expected to discuss his decision at the press conference, said Ryan's counselor Brendan Buck, who confirmed a wave of reports in a statement on Wednesday morning.
"This morning Speaker Ryan shared with his colleagues that this will be his last year as a member of the House. He will serve out his full term, run through the tape, and then retire in January," Buck said.
"After nearly twenty years in the House, the speaker is proud of all that has been accomplished and is ready to devote more of his time to being a husband and a father. While he did not seek the position, he told his colleagues that serving as speaker has been the professional honor of his life, and he thanked them for the trust they placed in him."
Political news site Axios, citing sources with knowledge of conversations between Ryan and his confidants, first reported Ryan's plan not to run.
In December, Politico reported that Ryan was considering retiring after the 2018 midterm elections, which polls and experts are suggesting is likely to break largely in Democrats' favor. A spokeswoman for Ryan said the speaker is "not going anywhere any time soon" at the time.
The White House said President Donald Trump "made sure that the Speaker knew, very clearly and in no uncertain terms, that if that news was true, he was very unhappy with it" in response to the December report.