One of the country’s top criminal lawyers failed to disclose hundreds of thousands of pounds of debts owed by her firm when she offloaded the business in 2016, according to allegations in a high court claim.

Karen Todner, who helped to fight US extradition requests on behalf of the alleged Nasa hackers Gary McKinnon and Lauri Love, is accused along with her advisers in a civil claim of hiding a series of liabilities, including allowing her former firm to write £275,000 worth of cheques which were then stored in an office drawer instead of being sent to payees.

According to court filings made by One Legal Services, the company that acquired Todner’s firm, Kaim Todner, three years ago, the concealed cheques had the effect of disguising unpaid debts and “but for such a practice the company would have appeared to be plainly insolvent”.

The allegations against Todner, who is contesting the claim, represent a blow to the public image of the solicitor, who is a former vice-president of the Solicitors Disciplinary Tribunal. She is best known for her work helping prevent the extraditions of McKinnon and Love to the US, where they were wanted on charges of hacking US government computer systems. Then home secretary, Theresa May, announced in 2012 that McKinnon would not be extradited, while Love’s appeal against extradition was upheld by the high court last year.

She also represented Tom Hayes, the Libor rigger, during his submission to the criminal cases review commission in 2017.

Todner’s business had suffered financial problems over a number of years. In 2012, her firm entered a company voluntary arrangement – a formal process enabling a compromise between a company and its creditors – and in February 2017 she was fined £2,000 and rebuked by the Solicitors Regulation Authority (SRA).

The regulator found that Todner had failed to notify the SRA about financial difficulties at Kaim Todner in 2012; failed to ensure that a client sought independent legal advice before she took a £25,000 loan from the same client in 2013; and allowed inappropriate payments to and from a client account, also in 2013.

While referring to the SRA fine in her defence to One Legal Service’s claim, Todner said that her firm had had cashflow problems, rather than being unprofitable, and that there was no need for independent advice in respect of the loan, which was repaid within six weeks. The use of the client account had no “adverse consequence”, she added.

In February 2016, Todner said Kaim Todner was to close; the business was saved a fortnight later by One Legal. Since that acquisition, One Legal says it has discovered hundreds of thousands of pounds in debts, on top of the £1.2m of total liabilities Todner and her representatives declared at the time of the sale.

One Legal claims in the court papers that the additional debts included:

• a further £252,000 owed to the Legal Aid Agency (LAA);

• a total of £148,000 that had been billed to the LAA on cases – including McKinnon’s – in excess of the legal aid cap.

Todner rejects the allegations and her defence papers state that One Legal was aware of the firm’s debts. She claims that there were only £47,000 worth of undelivered cheques, as the remainder related to bills that had been settled via a bank transfer, and that she had informed the purchaser as soon as she discovered her firm’s “practice of drawing cheques and placing them in a drawer”.

Todner added that the LAA was not owed £252,000 by Kaim Todner as that debt related to rolling contracts that did not require settling at the time of the sale and that she did not recognise the claims on legal aid cases.

She also said One Legal Service’s chief executive, Trevor Howarth, had told her that his companies “would never be instructed by the Police Federation” due to his “previous trial for perverting the course of justice and contempt of court”.

Howarth did not face a hearing for contempt of court, as the allegations against him, connected to a civil case, were thrown out by the court of appeal in 2014. Separately, he was acquitted by a jury in 2008 of allegations that he had perverted the course of justice during a speeding case, although he was subsequently criticised by a judge for being “slapdash”.

Todner has also issued a counterclaim alleging that One Legal Services agreed that she would not have to repay a £184,000 loan she received from Kaim Todner.

One Legal said its purchase agreement with Todner makes clear that she was due to repay the money.