Extraordinary details of how Boris Johnson allegedly overruled the advice of staff to promote the business interests of his former lover Jennifer Arcuri and win her affections are revealed in previously unpublished diary extracts by the US businesswoman.

According to one entry, the then London mayor even offered to be her “throttle” in an attempt to accelerate her business career, claims that may reopen the possibility of Johnson facing a potential criminal investigation into misconduct allegations.

The diary entries – which appear to have been written during Arcuri’s affair with Johnson and have been seen by the Observer – also suggest that he broke the rules governing ethical conduct in public office in his dealings with Arcuri.

The handwritten excerpts portray Johnson as desperate to offer help to her in promoting her fledgling business as he pursued a sexual relationship with the then 27-year-old.

One entry recalls how Johnson told her: “How can I be the thrust – the throttle – your mere footstep as you make your career? Tell me: how I can help you?”

Arcuri gave her diaries to the veteran journalist John Ware in 2019 after he made an ITV documentary on her relationship with Johnson.

At Ware’s request, Arcuri has now agreed to allow publication of some of the extracts following Johnson’s statements last week about public probity, including how MPs who break conduct rules “should be punished”.

Despite the prime minister’s comments last week, he never mentioned Arcuri in his declaration of interests when he was mayor, and after news of their alleged affair broke in 2019, he said there was no interest to declare.

The revelations will pile yet more pressure on the prime minister after 10 days of relentless allegations of sleaze and impropriety by Conservatives MPs, and growing anger inside the Tory party over Johnson’s own responsibility for, and handling of, the crisis.

The latest Opinium poll for the Observer today shows how the stream of damaging stories has hit Johnson and his party, with Labour now holding a lead over the Tories for the first time since January this year.

Boris Johnson, with Jennifer Arcuri, guest speaking at the Innotech Summit in July 2013.

The poll puts Labour on 37% (up 1 point), the Conservatives on 36% (down 1), the Liberal Democrats on 9%, the Greens 7%, and the SNP 5%. Johnson’s personal approval ratings have sunk to another all time low of -21%. A fortnight ago the Conservatives held a five-point lead over Labour.

Responding to the latest Arcuri revelations, a government spokesperson said: “As mayor, Boris Johnson followed all the legal requirements in the Greater London Assembly’s [sic] code of conduct at the time.”

The diaries, however, indicate that Johnson pursued Arcuri, offering to advance her business interests in the apparent hope that this might lead to a sexual relationship with the woman who dubbed him “Alex the Great”.

One diary entry, from 2012, states that Johnson told her: “I can barely control myself whenever I see you. You make me too excited. Baby I couldn’t wait. All year I have been waiting for you. All year. You drove me nuts. I have thought about no woman as I have thought of you.”

Potentially more damaging are excerpts that allege Johnson bragged about ignoring advice from his staff who urged him not to help Arcuri promote her tech company Innotech.

After Johnson had agreed to Arcuri’s request to be the keynote speaker at the launch of Innotech, Arcuri states in a diary entry for 27 February 2013 that Johnson boasted to her how he’d rejected the advice of his staff not to attend. It states: “I just want you to know they came to me and I crushed them. They said: ‘You can’t do this Innotech in April.’ I said: ‘Yes, I can, I’ll be there.’ I only want to do this to make you happy. How I do wish to make you satisfied.”

‘You are going to get me into so much trouble’, one diary entry alleges Johnson told Arcuri.

Another diary entry, this time from November 2012, alleges that Johnson told Arcuri: “You are going to get me in so much trouble.” She also claims that her lover admitted he was aware of a conflict of interest when she asked him to “validate” her tech work publicly.

Members of the Greater London Authority oversight committee which is currently investigating allegations of conflict of interest during Johnson’s time as London mayor called the revelations “significant”.

Committee chair Caroline Pidgeon, speaking in her capacity as an assembly member, said Arcuri’s diary notes were of serious concern. She said: “This new material from Jennifer Arcuri is significant and the IOPC [Independent Office for Police Conduct] may wish to consider whether they need to reopen their investigation.”

Last year the IOPC said it would not be launching a criminal inquiry into whether Johnson abused his position as mayor to “benefit and reward” Arcuri. Arcuri received £126,000 of public money in the form of grants for her technology business and event sponsorship. In addition, she was given access to foreign trade missions led by Johnson. Arcuri insists that none of them were granted personally by Johnson and to date there remains no evidence that they were.

IOPC investigators never had access to Arcuri’s handwritten diary entries in which she made “verbatim” notes of the highlights of his telephone calls and their conversations.

For the IOPC to open a new inquiry into whether Johnson should be investigated for criminal misconduct it must receive a referral from the GLA monitoring officer – an ethics watchdog – to look into the fresh allegations made by Arcuri in her diary. A GLA spokesperson confirmed its monitoring officer would assess “any new significant evidence” into the relationship between Arcuri and Johnson.

Although the police decided no investigation was warranted, the IOPC found his failure to declare the conflict of interest may have breached the GLA 2012 code of conduct.

At the same time as pursuing Arcuri for sex in 2012, Johnson endorsed the code in which he undertook not to bring the GLA “into disrepute” by using his position to “improperly confer on or secure for themselves or any other person, an advantage or disadvantage.”

In December that year, Arcuri returned to the US with Arcuri writing that Johnson was continuing to offer: “How can I be your footstool to your career?” She added how he was “always trying to think of ways to please me.”

Arcuri says Johnson never explained that their relationship posed a head-on conflict with the Nolan principles – ethical standards expected of public office holders and which promote “selflessness, integrity, objectivity, and honesty” in public life. She says that she’d never heard of Nolan until after news broke about their relationship in the autumn of 2019.