One said their ex-party leader should back the prime minister or “shut up”, adding she had “zero credibility”.
Mr Sunak is hoping to use the event in Manchester to convince voters he is the man to solve the UK’s problems. But senior Tories fear Ms Truss will instead remind the public of the economic chaos that defined her less than two months in office.
Ms Truss will speak at a “growth rally” at the conference, aimed at pushing Mr Sunak on tax cuts. Dubbed the Great British Growth Rally, organisers are already planning for a large turnout.
But Ms Truss’s plans have triggered fury among senior Tories, coming as they do when the party is facing a potentially disastrous electoral defeat next year.
Former minister Sir Alan Duncan told The Independent: “Liz Truss should back the prime minister or shut up.
“She has zero credibility and making unhelpful speeches is no way for a former prime minister to behave”.
Asked about Ms Truss, the former chancellor Lord Hammond said: "I definitely think that people who have done their stint and moved on should not use the conference to disrupt."
Another Tory grandee said the party had already suffered at the polls from years of factions and lack of discipline” and that for Ms Truss “as a former prime minister to take the stage advocating a different agenda [to Mr Sunak] is total and utter disgrace. How dare she? The tragedy is there is even one member of parliament thinks she is worth listening to”.
The row came as Mr Sunak’s recent relaunch had failed to work, he added, appealing to core voters rather than “expanding your appeal to other voters”.
Another senior Tory said that calls for Ms Truss to pipe down were “'naive optimism”.
An ally of Ms Truss said “colleagues and ex-colleagues taking potshots at Liz would do well to remember that she was the leader our grassroots members voted for last year. She has every right to be heard and to contribute to the debate over the agenda we need to embrace to win the next election.”
Former prime minister Boris Johnson, who has in the past drawn attention away from his party leader, is not thought to be planning to attend the conference.
In 2018 after he resigned as foreign secretary over Brexit, Mr Johnson caused mayhem at the Tory party conference, giving a crowd-pleasing speech to a capacity audience of 1,500, some of whom had queued for four hours to see him.
The Conservatives enter their last party conference before the election knowing their path to retaining the keys to Downing Street is narrow.
The party has been trailing Labour in the polls month after month.
Last week’s announcement on green legislation, including an extra five years before the phasing out of the purchase of new petrol and diesel cars, saw the Tories close the gap slightly.
But as the starting gun is fired on the race for power, Tory MPs are nervously watching Mr Sunak’s performance this week.
The prime minister is thought to have been working on his speech to conference for weeks.
Earlier this week he tried to play down the spectre of his predecessor, making fun of Ms Truss at a dinner with journalists.
“I’m glad to see actually that Liz Truss has been quietly reflecting. Not least on who’s to blame. The Treasury? Of course. Office for Budget Responsibility? Bank of England? In fact, I can’t think of a [body] in British public life that hasn’t yet been blamed.
“Except perhaps [her favourite think tank] the Institute for Economic Affairs.”