A British government minister has dramatically resigned over allegations he failed to act on claims of bullying in the Tories' youth wing.

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.