Grant Shapps said the "buck should stop with me" after allegations of bullying within the Tory youth group while he was Party chairman.
The International Development Minister stepped down after 21-year-old activist Elliott Johnson took his own life.
In his resignation letter, Shapps told the Prime Minister "neither I nor the Party could find any written allegations of bullying, sexual abuse or blackmail made to the Chairman's office.
"Over the past few weeks - as individual allegations have come to light - I have come to the conclusion that the buck should stop with me.
"Given the very serious nature of what has subsequently occurred I cannot help but conclude that the only right course of action is for me to step down as Minister in your government."
President Barack Obama said the US needs to crackdown on the "easy accessibility of weapons on our streets" after a shooting left three dead and nine injured at an abortion clinic in Colorado.
Paying tribute to the police officer who was killed in the shooting, Obama added: "May God bless Officer Garrett Swasey and the Americans he tried to save."
International Development Minister Grant Shapps is reportedly to resign from his post following complaints over his handling of allegations of bullying within the Conservative Party's youth wing.
The father of Elliott Johnson, a young activist who committed suicide called for Mr Shapps and Lord Feldman, his successor as party chair, to stand down after it emerged that bullying claims against campaigner Mark Clarke had been raised with the party before his son's death.
The party had previously said it could find no record of any complaints against Mr Clarke, but the Guardian since published a letter sent to the party in January - when Mr Shapps was party chairman - about another allegation of abuse.
Mr Johnson, who died in September, had named Mr Clarke in a letter left to his family after his death.
Asked about Mr Shapps' position earlier on Saturday, Prime Minister David Cameron had said a statement would be released later.
Police have named the suspect behind a shooting at a Colorado Springs abortion clinic as 57-year-old Robert Dear.
Andy Murray says he is confident Britain can win the Davis Cup as he and brother Jamie prepare to play their part in a doubles contest against Belgium on Saturday.
With the score currently tied at 1-1, the pair will be hot favourites to give Britain the lead in Ghent on Saturday.
Should they do so, victory for the British number one over Belgium's top seed David Goffin on Sunday would hand Britain its first Davis Cup title since 1936.
Speaking before taking to the court with his brother, Murray said: "I believe in myself. I believe in me and Jamie as a doubles team, as well.
"But it's obviously going to be tough. Goffin's a top-quality player when he plays well. I'm aware that will be a very tough match to win. In the doubles, Davis Cup is always tough, never easy, just because of the way doubles is played."
The British team were almost handed an unexpected boost on Friday when debutant Kyle Edmund took a two set lead in his tie against Goffin - but the Belgian came back to secure victory.
Murray's three-set win over Ruben Bemelmans meant the scores were tied going in to the second day.