Former Treasury head Lord Macpherson tweets that economic impact of Brexit should be "limited."

"Brexit is a risk but its economic impact should be limited provided [the government] seizes policy opportunity and looks forward not back."

Macpherson was a key part of so-called "Project Fear" — essentially an unproven Brexiteer belief that the pro-Remain establishment was deliberately trying to scare the British people into voting to stay in the EU.

LONDON — Lord Macpherson, the civil servant dubbed "The Architect of Project Fear," believes Brexit's negative impact on the British economy will be limited if it is handled correctly.

Macpherson was the Tresury's most senior civil servant before the Brexit referendum, and was responsible for the forecasts that suggested Britain striking a Canada style deal after a Brexit vote could lower long-term economic output by as much as 6%.

Those forecasts — as well as predictions of a possible Brexit induced recession — were held up by many on the Leave side of the referendum campaign as the cornerstones of so-called "Project Fear." "Project Fear" was essentially an unproven Brexiteer belief that the pro-Remain establishment was deliberately trying to scare the British people into voting to stay in the EU with unnecessarily downbeat predictions of the post-Brexit future.

Despite having been at the heart of the Treasury's gloomy predictions about the impact Brexit could have on the British economy, Macpherson is now a little more upbeat.

"Brexit is a risk but its economic impact should be limited provided [the government] seizes policy opportunity and looks forward not back," he said in a tweet a few days before Christmas.

Following up from the tweet, Macpherson told the Financial Times that "there is still all to play for."

"If the government relentlessly focuses on achievable outcomes in Brussels and pursues a sensible economic policy at home, it could yet keep any lasting damage from Brexit to a minimum. But that remains a pretty big ‘if’."

While the British economy has noticeably slowed since the vote to leave the European Union, it has confounded the most pessimistic forecasts. Predictions that the country would collapse into a recession after voting to leave the EU did not materialise, but neither did the forecasts from Brexiteers that the economy would continue to boom.

The UK's growth slowed markedly as uncertainty around Brexit pushed businesses to delay spending, and inflation caused by the weak pound made spending more expensive for consumers.