A former special adviser to the Welsh first minister, Carwyn Jones, who is facing an independent inquiry over the death of a sacked minister, Carl Sargeant, has claimed that a “toxic” atmosphere of “fear and loathing” existed within the Welsh Labour government.

Steve Jones, who was the first minister’s special adviser for five years, said Sargeant, who apparently took his own life following allegations of harassment, was upset by the “mind games” that went on within the government.

Steve Jones’s remarks follow allegations by the former Welsh minister Leighton Andrews that Sargeant was undermined from within the Labour government for several years.

An inquest on married father-of-two Sargeant, who was found dead at his home in Connah’s Quay, north Wales on Tuesday, four days after he was sacked, is due to begin on Monday.

Carwyn Jones also agreed to to launch an independent inquiry into his handling of the allegations, following criticism from Sargeant’s relatives. It is understood that three women made allegations, which his family have denied.

Steve Jones, who worked with the first minister and Welsh Labour leader from 2009 to 2014, said: “Soon after I arrived to work for Carwyn after his election as first minister, it was clear that I was entering a toxic environment. The atmosphere that existed on the fifth floor [the ministerial floor at Tŷ Hywel – a Welsh assembly building at Cardiff Bay] and in particular the first minister’s office, was one of fear and loathing.

“I’m no shrinking violet when it comes to politics. Prior to moving to Cardiff Bay, I worked as a special adviser to the government chief whip at Westminster. I was fully accustomed to the rough and tumble of politics at the very highest level.

“But the behaviour of some at the top of the Welsh government was like nothing I’d ever seen before. It was pure poison. Ministers were undermined by senior advisers playing power games and seeking to exert unreasonable control over government and [over] the first minister himself.”

Steve Jones said that Sargeant was among those who were badly treated. “Certain ministers, who were seen as a threat to these advisers, were frequently the subject of malicious rumours and concerted smear campaigns,” he said. “Those ministers, including Carl, would have their diaries unreasonably monitored and questioned, their policy proposals shelved and direct access to the first minister blocked. It went way beyond any ‘office politics’ or personality clashes.

“Dealing with our Plaid Cymru partners in government at that time was considerably more straightforward than dealing with the clique surrounding Carwyn.

“The mindgames being employed against ministers – such as Carl – were at times an utter distraction from the serious business of governing Wales.”

Steve Jones said that he was personally seriously affected by the culture he encountered. “After recognising the effect this behaviour was having on my own wellbeing, I informed the first minister of my intention to resign,” he said. “I told him precisely why I had decided to leave. He urged me to reconsider.

“He promised he would sort out any problems I had, if I tore up my letter. He convinced me he would address the situation; I agreed to remain.

“Things improved for a few months, then the poison returned and it began to engulf others – advisers and ministers alike.

Sargeant, he said, was particularly affected. “It was clear that all this was getting Carl down,” he claimed. “I told Carwyn that Carl was becoming increasingly unhappy. I suggested to him on more than one occasion that he should simply go for a pint with Carl one night – to talk about things and get back to the kind of relationship they enjoyed before and during Carwyn’s leadership campaign. I don’t know if they ever shared that pint.

“In any case, it became increasingly obvious that Carwyn was either unwilling or unable to address the culture that existed within his office. He allowed it to develop, fester and grow.”

A Welsh government spokesperson said: “We do not recognise these comments. All complaints regarding staff and special adviser conduct are taken seriously and dealt with accordingly.”

A spokesperson for Welsh Labour declined to comment.