The accountancy firm Deloitte has taken the extraordinary step of apologising to the government and offering to stop bidding for Whitehall contracts for six months, after preparing an internal memo suggesting Downing Street had no plan for Brexit.

Deloitte said on Tuesday night it had “put forward a plan” for working with the government that was intended to “put this matter behind us”.

No 10 has not denied reports that the strategy includes a six-month moratorium on pitching for lucrative government work.

It comes some weeks after the note, written by a Deloitte consultant who had worked for the government, was leaked to the Times. The note warned that cabinet splits were delaying the government’s ability to agree a negotiating strategy, which may not be ready for six months – well beyond Theresa May’s deadline of March 2017 for the launch of withdrawal talks with the EU.

It also identified “well over 500 projects” being undertaken by Whitehall departments to implement Brexit, creating the need for up to 30,000 extra civil servants.

At the time, No 10 said the document had no credence as it was nothing to do with the government. However, Deloitte’s decision to apologise and impose a punishment on itself underlines how much displeasure it appears to have caused in No 10.

The accountancy firm said: “Deloitte regrets the publication of the two-page note, and has apologised for the unintended disruption it caused government. The note was for internal audiences and was not a Deloitte point of view. We have put forward a plan for working with central government to put this matter behind us.”

The Deloitte note is one of a string of warnings about the scale of the task facing the civil service over Brexit.

Some of those highlighting the challenges are Gus O’Donnell, a former cabinet secretary; John Manzoni, the Cabinet Office permanent secretary; Simon Fraser, a former Foreign Office permanent under-secretary; Bob Kerslake, the former head of the civil service; and the Institute for Government.