US House of Representatives Speaker Paul Ryan will not run for re-election this year, in a big blow to Republicans ahead of autumn’s mid-term elections.
A spokesman for Congress’ most powerful lawmaker confirmed he would not stand for another term in his Wisconsin district this November.
A source close to Mr Ryan told the BBC his decision was family-related.
Republicans are already facing a tough challenge from Democrats to retain control of Congress’ lower chamber.
A statement from his adviser, Brendan Buck, said: “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.
“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 father.
“While he did not seek the position, he told his colleagues that serving as speaker has been the professional honour of his life, and he thanked them for the trust they placed in him.”
The Democratic Congressional Campaign Committee said Mr Ryan’s departure was an ominous sign for Republicans, months away from nationwide elections.
“Stay tuned for more retirements as Republicans increasingly realise that their midterm prospects are doomed,” the fundraising committee added.
Paul Ryan will go down in history as the Speaker who forced the most toxic agenda on the American people, then left them to clean up his mess.
Because even he sees the writing on the wall: The House is Democrats' for the taking. pic.twitter.com/tB423kN0vv
— DCCC (@dccc) April 11, 2018
The resignation of Mr Ryan – whose role as House speaker places him second in line to the president – will spark speculation about whether he could mount a White House campaign.
The clean-cut conservative, who has served in the House since 1999, was the vice-presidential running mate for Mitt Romney in 2012.
According to Axios, the 48-year-old father-of-three has found his job frustrating, partly because of President Donald Trump.
Mr Trump praised the speaker on Twitter, saying he “is a truly good man”.
“He will leave a legacy of achievement that nobody can question. We are with you Paul!” he added.
In December he achieved his long-held goal of overhauling the US tax code, and according to Axios, felt satisfied enough with that achievement to leave.
Reacting to the news, conservative congressman Jim Jordan told Fox News it was too early to decide who would replace Mr Ryan.
Steve Scalise and Kevin McCarthy are among the Republican congressmen whose names are being touted to step up as replacements.