Brexit will reduce Britain to a “mangy old lion” begging for trade deals from one-time colonies, head of the campaign group behind a drive for a second referendum has warned.
Lord Malloch-Brown, who chairs Best for Britain, said that the UK's loss of influence was exposed at last week's G7 summit, where Theresa May was left a "spectator".
And he said Vladimir Putin's Russia had identified the UK as Europe's "weakest link" to be targeted with fake news, political interference, dirty money and poison attacks.
In a speech in Oxford on Wednesday evening, Lord Malloch-Brown, a former United Nations deputy secretary general who served as foreign minister from 2007-09, will warn that Brexit has "ruined" UK foreign policy, leaving the country "all at sea".
He is due to say: "On foreign policy as on so much else, to stay and fight for a stronger Europe remains the only real path forward for a Britain confronted by a Europe and world in crisis.
"Far from running away from Europe, we should remember the real lesson of our history. Our security and prosperity is bound up first and foremost with Europe."
Lord Malloch-Brown will say that Britain was previously "America's pre-eminent link to Europe and Europe's intermediary with Washington".
But he will say that both relationships are now "in crisis" because of the result of the EU referendum and the election of US President Donald Trump.
"Britain has lost its anchors, one has been cut by Britain's own choice; the other by President (Donald) Trump," he will say.
"Theresa May appeared a spectator in the corner at last weekend's G7 summit in Canada. A stranger to Europe and to Trump."
He will add: "President Putin's Russia seems to have determined that Britain is Western Europe's weakest link.
"The crude attack in Salisbury; the interference in the Brexit referendum as well as more widely in British politics and media; and the penetration of Britain's financial and legal establishment by Kremlin-linked oligarchs. All seem to add up to a targeting of a Britain, isolated and vulnerable."
Meanwhile, Britain has been left "a supplicant at the doors of its former colonies looking for trade deals that aren't there", he will say, adding: "Talk about mangy old lions."