Grant Shapps, the Minister of State at the Department for International Development, has resigned after claims emerged he failed to act over allegations of bullying by a Tory campaigner.

In a resignation letter seen by the BBC, Mr Shapps told David Cameron "the buck should stop with me" over issues while he was Conservative Party chairman.

The Prime Minister earlier refused to offer his full support to Mr Shapps in the wake of the bullying allegations. He told reporters a statement would be made on the former party chairman today.

The father of Elliott Johnson, the Tory activist who is believed to have killed himself, said his son would still be alive if the party had acted on complains about Mark Clarke's behaviour.

He and the Conservative MP Ben Howlett claim successive party chairs, including Grant Shapps and the incumbent Lord Feldman, have been aware of Mr Clarke's reputation for years.

A letter emerged, written by former party chairman Sayeeda Warsi and sent to Mr Shapps, demanding action be taken against Mr Clarke. She said she never received a "satisfactory response".

The letter appeared to contradict the Tory party's claim it was unable to find records of uninvestigated complains against him.

In his letter to the Prime Minister, Mr Shapps writes: "Over the past few weeks - as individual allegations have come to light - I hvae come to the conclusion that the buck should stop with me.

"Given the very serious nature of what has subsequently occurred and my role in appointing Mr Clarke, I cannot help but conclude that the only right course of action is for me to step down as a Minister in your government."

Mr Clarke has denied any wrongdoing.

It had been reported Mr Clarke attempted to obtain photos of Conservative minister Robert Halfon leaving the East India Club with a woman who was allegedly his mistress, supposedly to blackmail the minister without portfolio.

Mr Halfon, who has a long-term partner but is not married, finished the affair in May, according to The Telegraph.

Mr Shapps courted controversy during his time as Tory party chairman.

He was forced to admit he had "screwed up" over his denial that he held a second job posing as a multi-millionaire under the pseudonym Michael Green while also an MP in 2006.

On the day of the 2014 Budget he sparked a backlash after he attempted to celebrate cuts in tax on beer and bingo to help "hardworking people do more of the things they enjoy".