The former UK chancellor, Philip Hammond, has been reprimanded by Westminster’s lobbying watchdog after he wrote to a Treasury official on behalf of a bank he was paid to advise.

In July 2020, Hammond emailed with a pitch from OakNorth Bank for a new “financial toolkit” that the government could use in response to the pandemic.

In the correspondence, obtained by The Daily Telegraph, the Conservative lord asked for the Treasury’s second most senior official to “pass it to anyone else who might be appropriate”.

Westminster’s lobbying watchdog, the Advisory Committee on Business Appointments (ACOBA), condemned Hammond’s conduct as “not acceptable” saying it was “an unwise step to contact senior officials at the Treasury on OakNorth’s behalf”.

The watchdog’s chair, Eric Pickles, said there was “a reasonable concern” that Hammond’s engagement with the Treasury “was only made available to OakNorth as a direct result of your time as chancellor”.

Letters reveal that Hammond tried to block the watchdog from publishing its ruling, and even considered a legal injunction. He demanded that the verdict be withdrawn and redrafted, telling Pickles: “What is appropriate or inappropriate is ultimately a matter of individual judgement… You cannot move the goalposts.”

But Pickles responded saying the decision had been final, and that Hammond’s behaviour had not been “in keeping with the purpose of the rules”.

The former chancellor left government in 2019 and signed up to OakNorth’s advisory board in early 2020.

When he was appointed to the OakNorth role, ACOBA ordered him to avoid contacting government ministers or officials for at least two years after the date on which he left office.

He was accused of breaching the Ministerial Code by breaking this ban last month, but defended his behaviour, saying: “My communication was neither seeking to influence policy, nor motivated by an attempt to secure business (or any other form of benefit) on behalf of Oaknorth or its partners/clients.”

Records show that Hammond quit his role at OakNorth in June, before allegations of lobbying first emerged.

At the time, Labour’s deputy leader, Angela Rayner, said: “Philip Hammond has broken the Ministerial Code, which is very clear about the conduct of former ministers. The breach needs to be investigated by the Cabinet secretary.

“Hammond has entirely disregarded the conditions that were made clear to him when he took the job with OakNorth Bank. If the rules are treated with such derision by the former Chancellor then the whole system is rotten.”

ACOBA’s ruling against Hammond is unlikely to have any real effect, as the watchdog has itself been criticised for being “toothless”. In April, openDemocracy revealed it had met only once since Boris Johnson’s election win in 2019.

Pickles has also faced heavy scrutiny in his role as the watchdog’s chair, and is one of 40 lords recently reported to Parliament’s standards commissioner over allegations raised by openDemocracy about their declaration of financial interests.

He was also accused of a conflict of interest after failing to publicly declare his role in a Conservative business lobbying forum, which boasts of its access to government ministers.