Tony Blair is to warn the Labour Party against "any attempt to whitewash" its general election defeat.
The former Labour Prime Minister says the party risks causing "irreparable damage" to its relationship with voters if it buries the reasons for its worst election result since 1935.
Following the result of last week's poll, Jeremy Corbyn said he would lead a period of "reflection" before standing down as Labour leader "early next year".
In a speech this morning, Mr Blair is to say: "Sure let's have a period of 'reflection', but any attempt to whitewash this defeat, pretend it is something other than it is, or the consequence of something other than the obvious, will cause irreparable damage to our relationship with the electorate."
Mr Blair, who is the only Labour leader to have won a general election in the last 45 years, will also unveil research by the Tony Blair Institute for Global Change, suggesting the party's problems "go deeper" than its Brexit stance and leader.
He will say that the party faces a crisis if it does not revitalise itself as a "serious, progressive" alternative to the Conservative Party.
"Labour needs not just a different driver, but a different bus," the report says.
"The first task is to discard the sectarian ultra-left politics that has taken the party over and condemned it to the wilderness of opposition. Only then can Labour begin the journey back to government."
talkRADIO: Listen live