Video report by ITV News Political Correspondent Libby Wiener
Former Prime Minister Sir John Major has urged people in three constituencies to vote for ex-Tory candidates rather than back Boris Johnson.
Sir John endorsed former Conservative ministers David Gauke, Dominic Grieve and Anne Milton, all of whom lost the party whip earlier this year when they rebelled over Brexit.
The three are now standing as independents in their constituencies, and Sir John said he would vote for them if he could.
"Let me make one thing absolutely clear: none of them has left the Conservative Party, the Conservative Party has left them," he will say later.
Gauke, Grieve and Milton were crucial votes when the Opposition passed the Benn act in September - essentially outlawing a no-deal Brexit. They were expelled from the party soon after.
"Without such talent on its benches, Parliament will be the poorer, which is why - if I were resident in any of their constituencies - they would have my vote," Sir John will say in a speech on Friday.
'Tribal loyalty has its place, but you need to vote with your head as well as your heart' - Sir John Major's video which will be played at the Stop The Brexit Landslide rally
Boris Johnson has described Sir John's view as "outdated, alas".
He said: "I think it's very sad, and I think that he is wrong... greatly though I respect him and his record."
Boris Johnson reacts to Sir John Major's endorsement of ex-Tory candidates at a news conference today.
Sir John will appear alongside ex-Labour Prime Minister Tony Blair in London at a rally pushing for a second referendum on Brexit.
Sir John describes the EU withdrawal as the "worst foreign policy decision in my lifetime", and it will affect "nearly every single aspect of our lives for many decades to come".
"It will make our country poorer and weaker. It will hurt most those who have least. Never have the stakes been higher, especially for the young.
"Brexit may even break up our historic United Kingdom," he will say.
Mr Blair is expected to say: "A no-deal Brexit is a risk no responsible Government would take, such a Conservative Party does not deserve to govern unchecked and the country would not be wise to let them."