The head of the official budget watchdog has defended its economic forecasts warning of the cost to Britain of leaving the European Union.
The Office for Budget Responsibility (OBR) infuriated pro-Brexit Conservatives with its prediction that withdrawal would wipe 2.4% off growth over the next five years while adding almost £60 billion to borrowing.
MPs such as former Cabinet minister Iain Duncan Smith and backbencher Jacob Rees-Mogg said leaving the EU would lead to a more liberal trade regime delivering a boost to the UK economy.
However OBR chairman Robert Chote said it was obliged to produce forecasts based on currently stated Government policy.
"The job that we have been given explicitly by Parliament is not to predict what we think the most likely outcome is going to be in the future but what the most likely outcome is conditional upon the current stated policy of the Government," he told the BBC Radio 4 Today programme.
"Obviously the outlook for policy as regards Brexit is not as clearly set out. We don't know what exactly the Government is going to be aiming for and what could be delivered in the negotiations on things like the trade regime, migration.
"We asked them whether they wanted to tell us any more about their policy in all of these areas than is already in the public domain and they said 'no'.
"Clearly it would have put us in a very difficult position if they had told us something and said we can't share that with the rest of the world."
Mr Chote said that as the likely shape of any post-Brexit settlement became clearer, it could feed into future forecasts but he warned that was unlikely to happen quickly.
"Will we end up in a world in which there are much lower tariffs both for us and for other countries? Are we going to end up with a very different looking migration regime? That remains to be seen," he said.
"For the time being we are left with the information that we have and that's what we can feed into the forecasts at this stage.
"We need the policy to clarify itself but we also shouldn't delude ourselves that it is going to clarify itself very quickly."