UPDATED: Shapps quits after claims that ‘failure’ to investigate party bullying led to activist’s suicide

Grant Shapps said he had come to the conclusion that the “buck should stop with me” amid a slew of claims about behaviour by Mark Clarke, who denies any wrongdoing.

The move came after the father of a Tory activist who killed himself called for the former party chairman’s resignation and that of current party chairman Lord Feldman, insisting his son Elliott Johnson would still be alive if they had acted properly.

In his letter to David Cameron, Mr Shapps wrote: “Although neither the party nor I can find any record of written allegations of bullying, sexual abuse or blackmail made to the chairman’s office prior to the election, I cannot help but feel that the steady stream of those who raised smaller, more nuanced, objections should have perhaps set alarm bells ringing sooner.

“In the end, I signed that letter appointing Mark Clarke ‘director of Road Trip’ and I firmly believe that whatever the rights and wrongs of a serious case like this, responsibility should rest somewhere.

“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.”

The Prime Minister heralded Mr Shapps’s departure earlier, describing Mr Johnson’s death as a “tragic loss”.

The 21-year-old was found dead on railway tracks in September, and apparently left a note condemning Mr Clarke.

The party had insisted it was not aware of any complaints against Mr Clarke, who ran the party’s “Road Trip” operation during the general election campaign, until August.

But former minister and Tory chairman Baroness Warsi disclosed on Friday night that she sent Mr Shapps a letter, dated January 20 this year, accusing Mr Clarke of publicly abusing her on Twitter.

“I look forward to hearing from you as to what action you intend to take against Mr Clarke,” it said.

The peer told the Guardian she never received a “satisfactory response” to her complaint, even though it was “common knowledge” that Mr Clarke was “a disaster waiting to happen”.

Mr Shapps said he had given Mr Clarke a “second chance” after he was removed from the party’s candidate list at the 2010 election.

“He presented himself as having learned from his past experience, being more mature and wanting to prove himself again,” he wrote.

“After some discussion, I appointed him in order to incorporate Road Trip into our wider campaign. The aim being to better co-ordinate his activity with our rapidly expanding Team 2015 target-seat operation.”

Mr Shapps said the Johnson family’s loss was “simply unimaginable”.

“More than anything, I am deeply shocked and saddened by the recent death of Elliott Johnson and my thoughts are with his friends and family,” he added.

In his letter accepting the resignation, Mr Cameron said : “I know that this will have been a very difficult decision for you to make, and something you will have given a great deal of thought to over recent days.

“I understand your reasons for stepping down and accept your decision.”

Mr Cameron said he would “always remember” that Mr Shapps had been “a loyal and trusted supporter of mine from the very beginning”.

“You have made a lasting contribution to the work of the Government, but you have also been a faithful servant of our party, and I know you have much more to give in the years ahead,” he added.

A replacement for Mr Shapps as international development minister will be announced later.

Attention is now likely to turn to the position of Lord Feldman, a key ally of Mr Cameron and one of the driving forces behind the Tory election victory.

Downing Street insisted the peer retained the Prime Minister’s “full confidence”.

Mr Johnson’s father Ray welcomed Mr Shapps’ resignation.

“It’s about time, he should have resigned several weeks ago,” he told the Guardian.

Speaking at a Commonwealth summit in Malta earlier, Mr Cameron said a “proper inquiry” was being held into the allegations and would be overseen by an independent lawyer.

“I feel deeply for his parents. It is an appalling loss to suffer,” he said.

“In terms of what the Conservative Party should do, there needs to be and there is a proper inquiry to ask all the questions and interview all the people who have come forward. That will take place.”

He added: “It is a tragic loss of a talented young life. It is not something that any parent should have to go through and I feel for them deeply.”