A distant cousin of Boris Johnson acted as guarantor for a line of credit of up to £800,000 while he was in Downing Street that helped finance his lifestyle.

Johnson had the backing of Sam Blyth, a millionaire Canadian businessman in the education sector, a second cousin of the former prime minister’s father, Stanley. It is understood that he used the credit line from February 2021 while he was at number 10, but did not draw down the full amount.

Johnson and his family are also believed to have stayed at a vacation villa owned by Blyth in the Dominican Republic.

Blyth does not appear to have any business interests in the UK, but he founded a Canadian chain of private schools known as Blyth Academy. He is an adviser to a Canadian global education company, ApplyBoard, where the international chairman is Johnson’s brother, former Conservative minister Jo Johnson.

The financial setup was revealed by the Sunday Times, which also reported that Blyth was on a list of headhunters’ recommendations for a chief executive position at the British Council, a non-departmental public body, around the same time. His name did not progress.

When asked about the deal, Blyth told The Guardian it was “far less” than £800,000 and “was done on fully commercial terms and fully approved in advance by the Cabinet Office and ethics.” “The prime minister did not ask or give anything for this,” he added. “And he didn’t know anything about the role of the British Council until I informed him that he had refused me to be a candidate.”

A Johnson spokesman said: “All of Boris Johnson’s financial interests are and have been duly declared. Boris Johnson sought the advice of the cabinet secretary, the independent adviser on ministerial interests and the ethics and decorum team. He followed his advice in full as the Cabinet Office has confirmed.”

He also noted that Johnson “did not take a loan from Sam Blyth.”

The spokesman added: “It is completely untrue that Boris Johnson has in any way assisted, or was aware of, any application by Sam Blyth, formal or informal, to fill any position on the British Council, and no one was in No 10 acting in her name.

“As far as he knows, no one at number 10 knew about this alleged app or did anything to promote it.”

It comes after lingering questions about how Johnson financed his lifestyle at number 10, after the furor over using a Tory donor, David Brownlow, initially to pay for the redevelopment of his Downing Street flat.

Johnson was known to be struggling with his £164,000 salary as premier, despite being provided with two homes: the flat and the use of the country retreat at Checkers.

Since leaving office, he has earned more than £1 million through speaking engagements, including to insurers and investors in blockchain, the technology behind cryptocurrency. He has also received a £1 million donation from a Thailand-based crypto investor, Christopher Harborne, to fund his office, fueling speculation that he is planning a political comeback, which his allies deny.

At the same time, his lifestyle continues to be subsidized by a wealthy Tory donor, with Anthony Bamford and his wife giving him the use of a £20m London home and a cottage in the Cotswolds. Johnson accepted the gifts despite co-owning three other houses, in Oxfordshire, London and Somerset.

In July, Lord Bamford hosted Boris and Carrie Johnson as they celebrated their lockdown wedding in the grounds of their 18th-century mansion, Daylesford House in the Cotswolds.

Johnson had abandoned plans, after his resignation as Prime Minister, to hold the celebration at the Prime Minister’s official country residence, Checkers, in Buckinghamshire. The “festival-style” celebration is said to have included a steel band, rum punch, Abba songs and a conga.