Liz Kendall launched a fresh attack on Jeremy Corbyn as voting closed in the Labour leadership election but insisted the party must accept the result if he wins.
The shadow health minister said the new leader will have won the right to pursue their agenda but made clear that she is likely to become a frequent rebel if the Islington North MP takes the top job.
In her final speech of the three month contest just 50 minutes before voting closed, Ms Kendall appeared close to tears as she warned that Ed Miliband's successor faces "huge challenges" to bring the party together after the "tumultuous and divisive" contest.
The Blairite candidate, who has been dubbed "Tory-lite" by some critics, conceded she may have been "too harsh" in the way she had set out her pitch.
"Perhaps I was too blunt, especially at the beginning when so many party members were still reeling form our terrible defeat," she said in a speech in central London.
"But my view is that in politics, as in life, you cannot deal with problems by ignoring or avoiding them. That it is never too soon to tell the truth."
Ms Kendall said that modernisers in the party must recognise that some members "doubt our principles altogether".
But she insisted Mr Corbyn would consign them to Opposition and again insisted she would not serve in his frontbench team.
"The programme Jeremy Corbyn offers is not new. His policies and politics are the same now as they were in the 1980s - and will end up delivering the same result.
"Neither is he the sole keeper of Labour's principles. No-one has a monopoly on being led by their conscience."
Ms Kendall insisted she would not "compromise my principles" on a number of areas including Britain's membership of Nato, the renewal of Trident or membership of the EU.
"I'm a person of principle. I think it would be a disaster for the country," she added.
"I have strong principles and Jeremy Corbyn is not the sole keeper of that. I want to be loyal to the Labour party whenever I can but there are some things that I believe are so important for the future of the country that I will do what my conscience tells me."
Ms Kendall told the Press Association that if she had appeared emotional during the speech, it was because she cared so much about the party.
"I feel I'm a tough and emotional woman because I care about these issues so much and I believe so passionately in our party and because I think it is our values and principles that are right for the country but we have got a lot of tough work to do it and I will never stop doing that," she said.