Confirmation from the OBR comes after a meeting with Prime Minister Liz Truss and Chancellor Kwasi Kwarteng on Friday.
A spokesman said the forecast “will, as always, be based on our independent judgment about economic and fiscal prospects and the impact of the Government’s policies”.
The meeting appears to have lasted less than 50 minutes.
An OBR statement reads: “We discussed the economic and fiscal outlook, and the forecast we are preparing for the Chancellor’s medium-term fiscal plan.
“We will deliver the first iteration of that forecast to the Chancellor on Friday October 7 and will set out the full timetable up to November 23 next week.
“The forecast will, as always, be based on our independent judgment about economic and fiscal prospects, and the impact of the Government’s policies.”
Richard Hughes, Andy King and Professor David Miles CBE – all members of the OBR’s budget responsibility committee – arrived at 11 Downing Street at 9.45am on Friday.
The decision to hold a meeting was welcomed by Conservative MPs and senior party figures, including former chancellor George Osborne, who oversaw the creation of the independent spending watchdog in 2010.
Calling it a “welcome move”, he said: “Turns out the credibility of the institution we created 12 years ago to bring honesty to the public finances is more enduring than that of its critics.”
Other sitting MPs expressed hope the meeting will mark the start of a process to win back the confidence of the markets, after the £45billion tax-cutting package announced by the Chancellor last Friday plunged the pound to historic lows and forced the Bank of England to intervene to calm the markets.
It launched an emergency Government bond-buying programme to prevent borrowing costs from spiralling out of control and stave off a “material risk to UK financial stability”.
Chairman of the Treasury Select Committee Mel Stride, one of a growing caucus of Conservatives with concerns about the Government’s plans, urged the meeting to be a “reset moment”, echoing earlier calls from fellow Tory MPs for a “plan B” from the Government.
Mr Stride was also among those calling on Mr Kwarteng to bring forward his planned statement setting out how he intends to get the public finances back on track after the OBR said it could produce a preliminary set of forecasts by October 7.
The Chancellor is scheduled to deliver his medium-term fiscal plan – explaining how he would get debt falling as a percentage of GDP, alongside the updated OBR forecasts – on November 23.
Chancellor Kwasi Kwarteng TOBY MELVILLE
Richard Hughes, Andy King, and Professor David Miles from the OBR arriving at Downing Street Gina Kalsi