Leaders of seven different parties have been invited to join a live TV debate in mid-May, but at least two of them have already ruled it out.
May has long claimed she would rather be "out and about" meeting voters, and Corbyn has ruled out participating without the Prime Minister.
It means viewers could be left with a "best of the rest" compilation, excluding the UK's two largest political parties.
As well as Labour and the Tories, ITV has invited the Scottish National Party, the Liberal Democrats, UKIP, Plaid Cymru and the Greens.
Despite the announcement, a Conservative spokesman told City A.M. that May's position remained "unchanged".
A Labour spokesman said: "Jeremy will not take part in an opposition leaders debate. The British people hae the right to see a head-to-head debate between the only two people who could form the next government - and the Prime Minister's refusal is a sign of weakness, not of strength."
TV debates are a relatively recent innovation in the UK's electoral cycle, having first been introduced in the 2010 contest, with David Cameron facing off against Gordon Brown and Nick Clegg. The Labour and Tory representatives infamously spent much of their first event declaring, "I agree with Nick".