Mark Sedwill, the UK’s former most senior civil servant, is to join the investment bank Rothschild & Co in his first appointment after leaving Whitehall this year.

Lord Sedwill quit as cabinet secretary, head of the civil service and national security adviser in June. His departure was seen as a victory for Dominic Cummings, the prime minister’s then most senior aide who had a tense relationship with Sedwill, as well as for Michael Gove, the cabinet minister pushing through a restructuring of government departments.

Rothschild & Co said Sedwill would be advising the investment bank on its “strategic ambitions” as well as helping to advise clients across its “global advisory, wealth and asset management and merchant banking businesses”.

Robert Leitão, the managing partner, added: “Lord Sedwill is recognised as one of the outstanding diplomats and public servants of his generation, and we are honoured that he has chosen to join Rothschild & Co as a senior adviser.

“Mark will bring a level of strategic perspective and measured judgment to enhance our group in the years ahead. His track record on the international stage speaks for itself and he will be a great asset to our business.”

Sedwill’s final months in Whitehall were marked by unnamed Downing Street sources claiming in national newspapers that he was failing to get a grip on the coronavirus crisis; some quoted in the Financial Times claimed he had fallen out with Boris Johnson over the response to the virus.

In April, Cabinet Office insiders told the Guardian the claims were “absolute crap”.

As cabinet secretary, Sedwill was in charge of coordinating the work of permanent secretaries as they grappled with the lockdown, supplies of personal protective equipment, food supplies, prison releases and coronavirus tests.