During PMQs yesterday, he said: 'She says it's about leadership, yet is refusing to defend her record in television debates and it's not hard to see why'.
"Once we get into a debate about the real issues - the policies about what's happening to the health service, education, the economy, care for the elderly and our future relationship with the European Union - I think you'll see the polls narrow and the prospect of a Labour government", Mr McDonnell said.
After the BBC and ITV confirmed they would have leaders' debate without her, Theresa May might join in after all.
A BBC spokesman said it was too early to say whether the broadcaster would put in a bid to stage a debate.
John McDonnell told Sky News that live debates during the Labour leadership contest had helped transform Mr Corbyn into "a candidate who could win" - and claimed the Prime Minister was anxious about history repeating itself in the General Election.
But after ITV confirmed it would host a leaders' debate and a senior BBC journalist said the corporation would not let a single politician stop a programme which was in the public interest, it is understood that Mrs May is now open to the idea of a Q&A session.
"That's what I have always believed in, it's what I still believe and I still do it - as prime minister, as a constituency MP, I still go out and knock on doors in my constituency".
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"We look forward to the straight fight between the Tories and the SNP in Scotland, but can the Prime Minister tell people why she is running scared of a televised debate with Nicola Sturgeon?".
The Scottish National party's leader in the Commons, Angus Robertson, also challenged May over the debates, asking: "If the prime minister is so confident of her hard-Brexit, pro-austerity, anti-immigration case, why won't she debate opposition leaders?"
"I think she's taken a judgment that she stands to lose more in doing them than she does in not doing them and she's probably right", said Natalie Fenton, at Goldsmiths, University of London.
Liberal Democrat leader Tim Farron told Sky News he was "very disappointed" that Mrs May was unwilling to attend any TV debate.
Both the BBC and ITV cited the fact that the debates had proved important in reaching sections of the electorate that tended to avoid other political media, particularly young people.
ITV is the first broadcaster to confirm a debate.
Although unpopular, observers consider May's decision to be a political calculation as she holds a roughly 20-point lead in the polls. Basking in the spotlight, his debate performances as party leader in 2010 and 2015 helped the Conservatives increase their seat count. Unlike in 2010 and 2015, Downing Street is more convinced of a majority vote and is understood to believe that it does not need the potentially unsafe TV debates to reach parts of the electorate that may be turned off by politics as usual.