A spokesman for Congress' most powerful lawmaker confirmed he would not stand for another term in his Wisconsin district this November.

Republicans are already facing a tough challenge from Democrats to retain control of Congress' lower chamber.

In a news conference with reporters, he said the decision was family-related.

"You all know that I did not seek this job. I took it reluctantly. But I have given this job everything I have no regrets whatsoever for having accepted this responsibility.

"But the truth is it's easier for it to take over everything in your life and you can't just let that happen."

He said he did not want to be known by his children as "only a weekend dad".

"Some of you know my story, my dad died when I was 16 - the age my daughter is - and I just don't want to be one of those people looking back at my life thinking I spent more time with my kids," he said.

• Profile: House Speaker Paul Ryan

Mr Ryan said that he will retire in January, after finishing his congressional term.

Mr Ryan - with his wife, Janna, and daughter, Liza, by his side - was re-elected comfortably in 2016

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.

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.

Fleeing a sinking ship?

Analysis by Anthony Zurcher, BBC News, Washington

Paul Ryan had to be coaxed into taking the speaker's gavel in 2015 and never seemed to relish the job. Rumours had been swirling in conservative circles for months that he was eying the exits.

The speaker will frame his decision as one of putting family first - and, with young children, the fundraising and legislative duties of the office are burdensome - but it's impossible not to view the move in a larger political context. He's had frequent clashes with Donald Trump, and their differences - in temperament and policy - continue to be stark.

A mid-term election looms, and there are at least even odds that a Democrat could be speaker next year. Even if Republicans prevail, their majority will certainly be diminished, making the job of passing legislation through a party already torn between moderates and ideological hardliners all the more difficult.

Rather than go down with the ship - or perhaps suffer the same fate as Tom Foley in 1994, the last sitting speaker to lose a re-election race - Mr Ryan is reserving his seat on a lifeboat. He's not the first congressional Republican to do so, and with the top man leading the way, more are sure to follow.

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.

• US House speaker deletes tax cuts tweet

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.