Philip Hammond has defended the estimated cost of Britain's departure from the European Union amid criticism the figure was "gloomy".
During the Autumn Statement, the Chancellor quoted figures from the Office of Budget Responsibility (OBR) to say the cost of Brexit could hit the British economy by as much as £60 billion.
Eurosceptics have hit out at the OBR forecasts, with Conservative MP Jacob Rees-Mogg describing the figure as "gloomy" and insisting post-Brexit Britain would be stronger as a result.
However speaking to Good Morning Britain on Thursday, Mr Hammond said the figure was "precautionary", and based on an economic period of "uncertainty".
He said: "Welcome to the real world: The world is full of uncertainties, how fast will the Chinese economy grow, what will Mr Trump do in America, what will exit from the European Union look like?
"Our economy faces a period of uncertainty as we negotiate our exit from the European Union and we've taken the judgement that with the economy predicted to slow down next year, it doesn't make sense to squeeze it harder.
"In the real world, whether you're running the Government's finances or investing in a business, you have to look across all these uncertainties and make certain judgements and prepare for contingencies."
"I've tried to take a precautionary basis, investing in Britain's productive capability, but setting the fiscal rules so it gives me a little bit in reserve to maintain jobs and security."
The OBR - which provides official independent forecasts of the Government's performance - showed the growth in Britain's economy will shrink from 2.2% to 1.4% over the next five years.
Shadow chancellor John McDonnell said the Brexit figure is "worrying" and was caused by a Government in disarray.
He told Good Morning Britain: "The Government can't tell us where they're going. They're in disarray.
"There's a £60bn gap with regard to Brexit - behind that there's another £60bn gap, which is about the mismanagement of the economy, and that has been masked."
Mr McDonnell called on the Government to open up about its Brexit plans, and blamed Theresa May's secrecy for worsening the economic damage from the referendum result.
"We need to know where you want to go, we need to know the strategy you're getting there and give certainty to the markets."
"Most of the problems that we're facing is because we've had six wasted years when we should have been investing."