Cabinet secretary urged to look into conflict of interest fears raised by Philip Hammond and Rachel Johnson

The UK’s most senior civil servant is under pressure to investigate Boris Johnson’s financial backers following cross-party claims that unnamed individuals stand to benefit from the prime minister’s willingness to pursue a no-deal Brexit.

John McDonnell, the shadow chancellor, has written to the cabinet secretary, Sir Mark Sedwill, asking if there may be a conflict of interest in Johnson’s acceptance of support from hedge funds that could gain from an economic shock.

Earlier on Saturday, Philip Hammond, the former chancellor, suggested Johnson was pursuing the interests of financial backers set to gain from a no-deal Brexit, in a major escalation of tensions in the prime minister’s own party.

Hammond said he was repeating a comment made last week by Rachel Johnson, the prime minister’s sister.

The former chancellor was accused by senior Tories of attempting a “smear” without evidence. However, Hammond was supported on Saturday by a series of MPs from across the Commons.

“Johnson is backed by speculators who have bet billions on a hard Brexit – and there is only one option that works for them: a crash-out no-deal that sends the currency tumbling and inflation soaring,” Hammond wrote in the Times.

Downing Street has refused to comment on the claim.

Guto Bebb, a former Tory minister who was thrown out of the party for opposing a no-deal Brexit, said: “The dubious financiers who supported the ‘leave’ campaign and the prime minister’s leadership campaign are betting against Britain. The PM should put the interests of the country first rather than facilitating a financial bonanza for a few.”

Anna Soubry, the leader of Change UK, said: “This week’s events are damning evidence that Boris Johnson has no moral compass. It gives me no pleasure to believe that Johnson is in hock to all manner of people and in particular those who don’t give a toss about the livelihoods of our constituents but simply get even more rich gambling on our children’s future.”

However, the business minister Nadhim Zahawi said Hammond was peddling a conspiracy. “Philip you let yourself down by claiming speculators on ‘no deal’ are interfering in us leaving the EU,” he tweeted. “This is an ugly smear that is completely untrue. If you know anything about finance or the City then you will know that.”

Hammond and his team have not named any individual donors. However, hedge fund managers have backed Johnson. One of those is Crispin Odey, a fund manager who has also previously backed Vote Leave and Ukip. Over the summer, it was reported that his fund had made a £300m bet against British businesses and stood to profit from an economic slump in the UK. However, the fund also backed other British companies. Odey did not respond to a request for comment.

McDonnell said: “Given widespread concerns raised by the former chancellor Philip Hammond’s comment today about Johnson’s speculator sponsors profiteering from a no-deal Brexit, I am writing to Mark Sedwill, the cabinet secretary, to seek a Cabinet Office investigation into this potential conflict of interest.”

The conflict of interest code for members of the House of Commons states: “Given that service in parliament is a public trust … members are expected … to fulfil their public duties with honesty and uphold the highest standards so as to avoid real or apparent conflicts of interests, and maintain and enhance public confidence and trust in the integrity of each member and in the House of Commons.”