Almost all of the investigations into alleged serious misconduct by Home Office staff relate to immigration matters, the Guardian has learned.

Although the department also has responsibility for policing and counter-terrorism, 96% of its internal investigations focus on immigration matters. Scores of these investigations have substantiated allegations against staff made internally or by the public.

The Home Office’s professional standards unit (PSU) investigates only the most serious misconduct allegations against teams or individuals within the department, or contractors such as those working in detention centres. A general complaints procedure deals with more minor matters that are not investigated by the PSU.

Areas of investigation by the PSU include allegations of crimes such as assault, sexual assault, racism, theft, fraud, harassment or “any behaviour likely to bring the Home Office into disrepute”.

The Guardian obtained a freedom of information response which revealed that in the past three years there have been 626 PSU investigations, with 210 allegations substantiated. Of the areas of the Home Office’s work investigated, 96.4% of the allegations related to borders, immigration and citizenship, with only a handful relating to other important areas such as policing and counter-terrorism.

The shadow home secretary, Diane Abbott, said the disclosures showed the Home Office’s immigration and nationality department was “not fit for purpose” and called for an immediate review.

The PSU has the power to request witness statements, CCTV footage, notebook entries, texts and voicemails on Home Office-issued mobile phones. Matters investigated may be referred to the police or other agencies where appropriate.

Home Office guidance states that the issues considered by the PSU have “potential for serious reputational damage to the Home Office as well as potential legal action taken against us. An assessment of each case will be conducted by the PSU to assess for the potential risk and if there is a risk of adverse publicity.”

In an answer to a parliamentary question last year, the then immigration minister, Caroline Nokes, revealed there had been 25 allegations of sexual assault made by detainees against staff in immigration removal centres between 2014-15 and 2017-18. She said in her response: “Any allegations of serious misconduct made by a detainee against staff in an IRC are also referred to the Home Office PSU for investigation.”

The Guardian requested information about recommended actions against individual members of Home Office staff contained in PSU investigation reports, but was rejected on the basis that each report would have to be examined individually and this would take more work than allowed under freedom of information request time limits.

However, the Home Office guidance states that the PSU’s Lessons Learned team tracks progress made on implementing recommendations in PSU reports and that there are quarterly Lessons Learned reports produced internally.

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Abbott said: “These figures show just how entrenched ‘hostile environment’ practices are in the Tory Home Office. The Windrush scandal has taught them nothing as they continue to rack up internal investigations with no real consequence and no substantial change. This culture is destroying lives and families every day and cannot be allowed to continue. The immigration and nationality department of the Home Office is clearly not fit for purpose, and the government must call an immediate review into its continued failings.”

Toufique Hossain, a director of public law at Duncan Lewis, which has obtained disclosure of some of these internal PSU reports as part of legal challenges against the Home Office in immigration matters, said: “Grave concerns are raised as to Home Office failings on a daily basis. A government body essentially investigating itself, put simply, will never hold itself accountable.

“The burden is very much placed on the individual under the Home Office’s control, more often than not with the assistance of publicly funded lawyers and NGOs, to ensure that the Home Office is held to account. Only through these mechanisms can vulnerable individuals access courts in order to vindicate their rights.”

A Home Office spokesperson said: “We expect the highest levels of integrity and professional conduct of both staff and contractors. The number of complaints investigated by the professional standards unit has fallen by more than 25% since 2016-17.

“Borders, immigration and citizenship system teams have the greatest interaction with members of the public so it is to be expected that there are higher numbers of complaints than for other non-public facing Home Office teams.”