Youngs out of Lions tour
He did so because of his sister-in-law's terminal illness, the England scrumhalf's club, Leicester, announced Saturday.
The wife of Ben's older brother Tom, himself a Leicester, England and Lions hooker, has been suffering from cancer for three years.
Saturday's statement from Leicester said: "the family has now received the devastating news that the illness is terminal".
Later Saturday, the Lions announced that Scotland scrumhalf Greig Laidlaw would take Ben Youngs's place.
Ben Youngs made two Test appearances for the Lions during their victorious tour of Australia four years ago, with Tom, now 30, featuring in all three matches of the 2-1 series win over the Wallabies.
Ben, now 27, was selected in coach Warren Gatland's 41-man party for the combined side's three-Test series away to world champions New Zealand on the back of starring for Six Nations kings England this season.
But he informed the New Zealander of his decision to withdraw this weekend.
"We are a very close family and, as I am sure everyone can respect, time is now precious together," said Ben Youngs.
"The most important thing for me at this difficult time is to be able to offer as much support as I can to Tom and his family in the remaining time we all have together," added Youngs, who has won 70 England caps.
"It is a great honour to be selected as a member of the British and Irish Lions squad and I would like to thank Warren Gatland, the staff and the other players for their understanding in this situation.
"I wish them well and, like all rugby supporters in British and Ireland, hope they have a successful tour."
In a separate statement, Gatland, again on secondment from his role as Wales coach, said: "We fully understand and respect Ben's decision to stay at home.
"Family comes first and I know from having toured with Tom and Ben in 2013 how close they are.
"This is a difficult and important time for them and we send Ben, Tom and their family our heartfelt thoughts."
Saturday saw Leicester secure their place in the title-deciding English Premiership play-offs and the Tigers said both Youngs brothers would still play the remainder of the domestic season for the Midlands club.
"Both families hope their privacy can be respected at this difficult time," said the club statement.
"There will be no further comment from Tom, Ben or from Leicester Tigers at this point."
Laidlaw, a veteran of 58 Tests and a proven goal-kicker, recently returned to club duty with Premiership side Gloucester after a knee injury suffered in January cut short his Six Nations campaign.
The 31-year-old Laidlaw, whose uncle Roy, an outstanding Scotland scrum-half, toured New Zealand with the 1983 Lions, was left out of the squad announced by Gatland last month, after the native New Zealander picked Youngs, Ireland's Conor Murray and Wales's Rhys Webb as his original No 9s.
At the time, Gatland stressed that Murray, his likely first-choice scrumhalf against the All Blacks, had to prove his fitness before getting on the plane after suffering a neck/shoulder nerve injury playing for Ireland nearly two months ago.
Saturday witnessed Murray's first appearance since being sidelined as he played 28 minutes of Munster's 50-14 Celtic League victory over Irish rivals Connacht at Thomond Park.