The OBR adds that the need to increase defence spending due to the war in Ukraine could add to the economic hardship.
It would take £40billion in tax rises or spending cuts for 10 years to get debt back to pre-Covid levels, the OBR claims.
Chief Secretary to the Treasury Darren Jones said: “It has laid bare the shocking state public finances were left in by the previous government.”
But Shadow Treasury Minister Gareth Davies claimed Labour “are fabricating a narrative to make way for reckless tax rises.”
Mr Starmer has hinged much of his economic planning on increasing growth in the UK.
However, figures from the Office for National Statistics (ONS) showed this week that the UK economy flatlined in July.
This was the second month in a row that the economy saw no growth, despite the fact economists were anticipating a recovery.
In response to the news, Chancellor Rachel Reeves said: “I am under no illusion about the scale of the challenge we face and I will be honest with the British people that change will not happen overnight. Two-quarters of positive economic growth does not make up for 14 years of stagnation.
“That is why we are taking the long-term decisions now to fix the foundations of our economy.”