In his resignation letter minister told the PM ‘the buck should stop with me’ after it emerged he was asked to act years ago over Mark Clarke’s behaviour

Grant Shapps, a former Conservative party chairman, has resigned from the government in disgrace following revelations that he had been warned about bullying in the party before the death of one of its young activists, who is believed to have killed himself.

He was forced to quit as minister for international development after the father of 21-year-old Elliott Johnson claimed that his son would still be alive if Shapps and other senior figures had behaved “responsibly” when made aware of the behaviour of one of the party’s organisers.

Johnson was allegedly targeted by Mark Clarke, who ran the party’s “road show” during the election campaign and was responsible for ferrying activists around the country. Shapps was responsible for Clarke’s appointment to the role. A string of bullying allegations levelled at Clarke have since emerged. In an exchange of letters with David Cameron, Shapps continued to maintain his innocence in the affair, but tendered his resignation because “responsibility should rest somewhere”.

“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,” he wrote. “In the end, I signed that letter appointing Mark Clarke ‘Director of RoadTrip’ 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.”

Facebook Twitter Pinterest Elliott Johnson was found dead on railway tracks. Photograph: Facebook

Johnson, a blogger for a Tory pressure group, was found dead on railway tracks at Sandy station, Bedfordshire, on 15 September. In a letter, he accused Clarke, nicknamed one of the “Tatler Tories” for his one-time appearance in an edition of the magazine, of bullying him. Until this weekend, Conservative central office had repeatedly claimed it had no warning of Clarke’s alleged behaviour before Johnson’s death. However, on Saturday the Guardian revealed that Tory peer Sayeeda Warsi had written to Shapps, her successor as party chairman, in January to demand action against Clarke for abusing her on Twitter, but had not got a satisfactory response. She said Clarke was a “disaster waiting to happen and this was common knowledge”.

The scandal appeared to move a step closer to Cameron’s door after it emerged that he wrote to Clarke to thank him for his work during the election, gushing: “We quite simply could not have done it without you.”

Amid the gathering storm for Cameron, the first sign of Shapps’s downfall came at a press conference on Saturday in Malta where the Commonwealth heads of state are holding a summit. Repeatedly asked to offer Shapps his full support, Cameron pointedly refused.

He said: “On Grant Shapps, there will be a statement made later on today about that issue. I think it is important in the tragic case that took place, that the coroner’s inquiry is allowed to proceed properly. The Conservative party has an independent inquiry under way under the oversight of a senior legal figure. I feel deeply for his parents. It is an appalling loss to suffer, and that’s why it is so important that there is the proper independent inquiry.

“There needs to be, and there is, a proper inquiry to ask all the questions and interview all the people who come forward and that will take place. There is an independent lawyer from Clifford Chance, who will oversee that process and make sure that it reaches clear conclusions from the evidence that comes through.”

Asked to respond to Johnson’s parents’ demands for Shapps’s resignation, he said: “What I would say is that it is a tragic loss of a very talented young life and it is not something that any parent should have to go through, and I feel for them deeply. What the Conservative party must do, and is doing, is ensure that there is a proper investigation into this issue, into the allegations that were made and who they were made to and all the rest of it, and that is why it is being overseen by a senior lawyer from Clifford Chance and we will act on the findings of that inquiry.”

Attention will now fall on Lord Feldman, a close friend of Cameron, who was co-chairman with Shapps at the time of some of the alleged bullying. However, a spokesman for the prime minister said that he had full confidence in Feldman, who is still chairman of the party.

In his resignation letter, Shapps wrote that he had given Clarke – who in 2010 was the subject of complaints about his behaviour – a “second chance”.

Campaign organiser Mark Clarke with David Cameron.

He wrote: “In July 2014 I gave a second chance to former candidate Mark Clarke. Having been removed from the candidates list following the 2010 election, Clarke had gone on to establish a campaigning organisation called RoadTrip2015. He presented himself as having learned from his past experience, being more mature and wanting to prove himself again.

“After some discussion, I appointed him in order to incorporate RoadTrip into our wider campaign. The aim being to better coordinate his activity with our rapidly expanding Team2015 target-seat operation. During this summer and autumn, there have been widely reported, very serious allegations made about the conduct of this former activist, who has subsequently been expelled from the party. I appreciate that there are ongoing coroner and party investigations under way, yet whatever the outcome of these processes, I doubt any of this will bring much comfort to the Johnson family whose loss is simply unimaginable.”

Elliot’s father, Ray Johnson, speaking at home in Wisbech, Cambridgeshire, said: “It’s about time. He should have resigned several weeks ago. It’s typical of these politicians, they cling on to the greasy pole for as long as they can. Eventually, they get kicked off or dragged off, the fact that they hang on does no one any favours. It makes them look less dignified.

“It justifies our view of Grant Shapps. But there are others involved and we need to ensure there’s a clearout of all these unsavoury characters at CCHQ.

“There are other people involved in this scandal and we’ll take one pin down at a time if need be.”

In Cameron’s formal response to Shapps’s letter, he said he had made a “big contribution” and had “much more to give in the years ahead”.

Clarke has denied all the allegations against him.

Nick Hurd was appointed to replace Shapps at the Department for International Development.