It will be an unwanted distraction for the embattled agency, which faces questions over its very future ahead of a hearing with the PRCA on Friday over its activities with a controversial South African client.
Chapman, who is currently holidaying in Greece and is expected to return to the agency next week, joined Bell Pottinger in late June from a role as Davis' chief of staff.
While his Twitter following was already well into five figures when he arrived at the firm, it has now swollen to more than 52,000 following a lengthy series of comments in recent days, which have included the suggested creation of a new anti-Brexit political party, which he suggests calling The Democrats.
These have been picked up by multiple news outlets, with varying degrees of amusement, curiousity and shock - with Chapman himself tweeting a Times report on his behaviour.
David Davis aide James Chapman launches online onslaught with 250 tweets | News | The Times & The Sunday Times https://t.co/X4pOlBCjVd— James Chapman (@jameschappers) August 15, 2017
Yesterday, an article appeared on political website Guido Fawkes claiming that Chapman was having a breakdown live on Twitter, and implying that he had been "eased out" of a former role as political editor at the Daily Mail before being left in limbo by the Conservatives after the sacking as Chancellor of ex-boss George Osborne.
Characterising his tweets as "Trumpish", the article said Chapman had compromised his new job at Bell Pottinger.
It said: "His ability as a lobbyist counselling clients specifically on Brexit may be a little hindered by his very public and frequent meltdowns where he has attacked almost all the people clients might hope he had some influence with; government ministers, newspaper editors, not to mention influential journalists who write about his clients," the article said.
The piece also carried tweets by politicians and journalists who said they were worried about Chapman’s mental health, and had been advised to "go easy" on him, following tweets aimed at numerous politicians and journalists.
Chapman's targets included former employers and other prominent public figures.
He signed off many of his tweets with a call to action to attend a rally in Parliament Square on 9 September. He challenged Theresa May to debate him on the Government’s Brexit plans there.
Chapman dismissed the Guido article as "dark arts" and said he himself had "learned from the BEST", namely the Daily Mail, before continuing with his social media rant.
Tweets today have included attacks on Prime Minister Theresa May.