Lord Peter Hain has called for rational thought in regard to the Organisation for Economic Co-operation and Development report which was made public on Tuesday.
In a meeting with Chancellor Philip Hammond, the OECD said it forecasted that the negative impacts of Brexit could be reversed with a major political decision - like a second referendum.
It warned a slow decline could occur if Britain distanced itself from the EU, and called for the country to call upon the spirit it took during the Blitz to get through.
Hain, a former Labour MP for Neath and a minister under Tony Blair, said the OECD is a "highly respected" organisation.
He told Julia Hartley-Brewer: "There’s a human cry among Brexiteers like yourself that they have the gall to express their point of view.
"[But the OECD[ retain a lot of expert economists, and I think we should take them seriously.
"We need to look at this coolly and rationally, rather than think it’s a traitorous event for a Paris-based think tank to express concern about the consequences of Brexit when most British commentators are saying the exact same thing.”