SINGAPORE - Britain's decision to leave the European Union (EU) was a "historic mistake" that will weaken both sides economically and politically, said Sir John Major.

The former British Prime Minister was speaking at the Credit Suisse Global Megatrends Conference 2017 on Thursday morning (April 20) at the Fairmont Singapore.

Sir John, who served as British Prime Minister from 1990 to 1997, delivered a scathing criticism of Brexit and outlined the harsh realities of the complex negotiations ahead in his keynote address at the conference.

His comments come days after British Prime Minister Theresa May called for a snap election on June 8, the outcome of which could shape the country's relations with its closest neighbours for decades to come.

Britain's exit from the EU will weaken the latter significantly especially against other global superpowers like the United States and China, Sir John said. Britain is one of Europe's largest economies as well as one of the few European powers with nuclear capabilities, he noted.

"Europe, the cradle of modern Western civilisation, is about to become less relevant."

He added that Brexit will contribute to the EU's mounting internal challenges, including the rise of populist sentiment across the region.

Britain has strong ecoomic and political institutions and is unlikely to face economic catastrophe upon leaving, Sir John said. But it will face challenges in the long-term.

"I don't fear for the UK's economic future...What I do fear is that the UK will be less influential politically and do less well economically."

The economic downsides will be felt in the long-run, for instance in the form of slowing investment growth, he noted.

Britain is now also facing long, complicated trade negotations with the EU as well as with other trading partners across the world - the outcomes of which are less than certain, Sir John said.

But ultimately the UK and EU will have to continue engaging with each other regardless of the outcome of Brexit negotiations.

"Whether this gamble was wise or foolish, only time will tell," he added.