Dominic Cummings, Boris Johnson’s most senior aide, is facing conflict of interest accusations over a publicly undisclosed consultancy job at a healthcare start-up endorsed by the government and in pole position to receive cash from a £250m NHS fund.

Cummings advised Babylon Health, a controversial AI start-up, on its communications strategy and senior recruitment just months before its “GP at Hand” app was publicly backed by Matt Hancock, the health secretary, a joint investigation with The Guardian can reveal.

Cummings’ paid role with Babylon concluded in July last year but he continued to advise the company until September 2018. That month Hancock visited the company and told staff he wanted the NHS to help Babylon expand.

In August this year, shortly after Boris Johnson entered No 10 with Cummings as his top adviser, Downing Street and the Department of Health announced a new £250m fund for boosting the use of artificial intelligence for diagnoses and data analysis in the NHS. Although the money has yet to be allocated, Babylon welcomed the announcement.

There are no rules requiring special advisers to disclose previous private sector roles, but MPs said Cummings’ undisclosed advisory role raised serious questions about potential conflicts of interest.

“The links between Dominic Cummings in the heart of Downing Street, the health secretary and this AI health firm are increasingly murky and highly irresponsible," the shadow health secretary, Jonathan Ashworth, said.

“Mr Cummings’ work for this company raises serious questions about a potential conflict of interest given the firm could be in line to receive public money from this new £250m AI fund.

“We need to know if he declared his work for the firm to the cabinet secretary when he joined the government payroll.

“Cummings should come clean on any other consultancy work he did before entering Downing Street so the public can have confidence that there are no other conflict of interest issues at play.”

Sarah Wollaston, the Liberal Democrat chair of the health select committee, called for reforms to the rules governing the conduct of special advisers.

“We need greater transparency on the potential conflicts of interest for all special advisers, especially given the scale of their power and influence,” she said.

Babylon confirmed Cummings was paid via his company, Dynamic Maps Ltd, which he set up in October 2017 as the sole director.