She said she was not interested in politics until Mr Corbyn was elected.
Britain's Conservative party will win a "Margaret Thatcher style" landslide in next month's national election unless the Labour party improves its poll ratings, according to Tom Watson, Labour's deputy leader.
With some moderate Labour MPs reported to be in revolt over Mr Corbyn's radical programme of renationalisation and expanding public services, Mrs May will accuse him of a return to the "disastrous socialist policies of the 1970s".
But he sowed confusion over the future of the UK's nuclear deterrent by suggesting it would form part of strategic defence review (SDR) under a Labour government.
Asked about those comments, Mr Johnson replied: "Whatever you may think of the American administration, it is vital we have strong relations with Washington".
Corbyn, a longtime anti-war activist who opposed the invasions of Afghanistan and Iraq, used a speech at the global affairs think tank Chatham House to outline his vision for defense and foreign policy.
He said the world was "more unstable than even at the height of the Cold War" because of a failed approach to worldwide security, with botched foreign interventions making the world a "more risky place".
"We are well on target for there to be £2m-plus bets on this general election, which would be a record for a single political event beating the Brexit referendum and Donald Trump unexpectedly beating Hillary Clinton to become United States president", the spokesman added. "But I am not a pacifist", he said.
Corbyn opposed military action in Iraq and Afghanistan among other recent conflict and will use his speech to brand the so-called War on Terror a failure. They have not increased our security at home - just the opposite.
"I accept that military action, under worldwide law and as a genuine last resort, is in some circumstances necessary", he said.
The expectation is that nearly all of those seats would come from the main opposition Labour Party, with the Liberal Democrats gaining few and the Scottish National Party losing a handful, it said.
Throughout the speech, Corbyn made clear that a vote for Labour was a vote against foreign intervention. Former deputy prime minister Nick Clegg told a Brexit conference in London: "It is a delight to be here on a Saturday morning in the middle of possibly one of the most listless, soulless and dreary general election campaigns I can ever remember".
He said: "The best defence for Britain is a government actively engaged in seeking peaceful solutions to the world's problems".
"We all want peace, but you can't take tea with terrorists who order attacks on innocent civilians on our streets".