The news that the longtime accounting firm for the Trump Organization has cut ties with the company and retracted 10 years of its financial statements is a new and serious blow to Donald Trump’s increasingly frenzied battle to fend off the legal investigations that are rapidly engulfing him.

The revelation that Mazars USA last week ended its relationship with the Trump family comes at a perilous moment for the former president as he strives to protect himself, his family and his business from legal threats that are now coming thick and fast.

A Guardian tally this month found that Trump was facing a total of 19 legal challenges, six of which involve alleged financial irregularities.

By withdrawing its stamp of approval from the documents, Mazars leaves Trump potentially exposed to substantial legal and financial trouble.

The papers, known as statements of financial condition, were used by Trump and his family business to attract and secure hundreds of millions of dollars in loans. They are also at the centre of an escalating investigation by the New York state attorney general, Letitia James.

Last month James tightened the screws on Trump and the Trump Organization by releasing details in a filing of several instances involving golf courses, real estate and other assets where the family had allegedly “falsely and fraudulently valued multiple assets and misrepresented those values to financial institutions for economic benefit”.

In a letter dated 9 February, Mazars’ general counsel, William Kelly, told the Trump Organization that the annual financial statements it had prepared for the family business between 2011 and 2020 were no longer reliable.

The accountants said they had based their decision partly on their own investigation into Trump’s finances and on the “totality of the circumstances”, concluding that “we are not able to provide any new work product to the Trump Organization”.

On the back of James’s latest attack, Michael Cohen, Trump’s former personal attorney and an ex-vice president of the Trump Organization, told the Guardian that in his opinion “the House of Trump is crumbling”.

James’s investigation is one of the most advanced and potentially dangerous of all the 19 legal actions bearing down on Trump. The inquiry is being pursued on both civil and criminal lines.

James is working in tandem with a separate criminal investigation by the Manhattan district attorney, Alvin Bragg. That inquiry is also looking into whether Trump and his family concern defrauded lenders or underpaid taxes by falsely representing his assets.

The disclosure that Mazars had broken off relations with Trump was included in a new court filing from James on Monday as part of her ongoing attempt to force Trump and his two eldest children, Donald Jr and Ivanka, to testify under subpoena.

Trump has consistently denied financial impropriety and has attempted to cast doubt on James’s investigation by denouncing it as a partisan witch-hunt. James is a Democrat, while Trump won the presidency in 2016 as a Republican.

The Trump Organization said it was “disappointed” by Mazars’ decision but tried to spin the development in a positive light. It selectively cited a line in the Mazars letter that said that “we have not concluded that the various financial statements, as a whole, contain material discrepancies”, adding that the comment rendered the James and Bragg investigations “moot”.

As Trump’s legal and financial woes deepen, he is also being assailed by a flurry of bad news surrounding the congressional investigation into the January 6 insurrection at the US Capitol. Trump, who is at the centre of the House select committee inquiry given that his “big lie” that the 2020 election was stolen from him drew thousands of his supporters to the Capitol building that day, has been trying to persuade his closest advisers not to cooperate.

This week it was revealed that John Eastman, a conservative law professor who was integral to attempts to persuade the then vice president, Mike Pence, to delay certification of Joe Biden’s victory on January 6, has handed over 8,000 pages of emails to the committee.

It has also become known that Rudy Giuliani, who as Trump’s lawyer was a key figure in the campaign to overturn the presidential election results, has opened a dialogue with the committee that could see him testifying in some form.