Labour leadership contender Liz Kendall will try to curtail the apparently unstoppable surge in support for Jeremy Corbyn by warning that his left-wing politics are “not right for the party or the country” and his victory would be a “betrayal” of voters.

In a forthright interview with The Independent on Sunday, Ms Kendall says those who are backing the MP for Islington North are “looking for easy solutions but they’re the wrong solutions” and that his policies “won’t change the country for the better”. She also points out that Mr Corbyn has failed to rule out voting for Britain to leave the EU, which she says would be a “catastrophe”.

The MP for Leicester West says that in the event of a Corbyn victory she would not only refuse to serve in his shadow cabinet, but would fight to keep “the party I love” from splitting in two – a suggestion that she would back a coup to oust him. Other shadow cabinet members have told the IoS they would fight to keep the party united, despite claims by some, including the Labour donor John Mills, of an SDP-style breakaway after a Corbyn victory.

Ms Kendall attempts to recast the leadership race, which has 48 days left to run, as a straight two-way battle between herself and Mr Corbyn, who has stunned Westminster by securing the most constituency party nominations and coming top in a YouGov poll of members.

Apart from Mr Corbyn, she says, “I am the only other candidate who is prepared to make the arguments for what I believe in”. However, she says, in contrast to him she is promoting “politics that will see us get elected and change the country for the better”.

She also attacks aides to the other two candidates, Yvette Cooper and Andy Burnham, for “unattributable briefings” about her having no children, and criticises the two MPs for not being clear from the start that they would not serve in a Corbyn shadow cabinet.

Ms Kendall also admits she has “lady balls” after showing resilience in the face of accusations of being a “Tory” at hustings and telling a male journalist who asked about her weight to “f*** off”.

She insists she is having a “great time” in the contest, despite being attacked on social media and at hustings for her views, and questioned about her private life. Despite her previous claims that she is not a Blairite, the MP repeatedly refers to Labour’s mistakes of the “past eight years” – a thinly veiled reference to the period since Tony Blair left office, rather than the five years since Ed Miliband won the leadership.

She says she is “not surprised” by the surge in popularity for Mr Corbyn because “party members are feeling they want a change”. Referring to herself and her left-wing rival, she adds: “There are two people offering a distinct change from where we’ve been over the past eight years. We’ve been through this awful defeat and trauma and many people are looking for easy solutions, but they’re the wrong solutions because they’re the politics of the Eighties that leads to the results of the Eighties and the early Nineties.

“Many party members have found it tough to take on the Tories with a credible alternative, and many people want to see the party move even more to the left. But that’s not going to get the Tories out and the policies that Jeremy is putting forward won’t change the country for the better.”

Referring to Mr Corbyn’s refusal to rule out a No vote in the EU referendum, Ms Kendall says: “I think that would be catastrophic for Britain’s future and the future of young people.”

Jeremy Corbyn, the surprise frontrunner for next Labour leader (Getty)

Asked whether she agreed with Mr Blair’s remarks last week that supporters of Mr Corbyn needed a “transplant”, Ms Kendall says: “That wouldn’t be the language that I would use but we want to put our values into practice – that means beating the Tories and setting out an alternative for 2020-25, not of 30 years ago.”

If Mr Corbyn wins on 12 September, she says: “We will be out of power for a generation and that will be a betrayal of all of the people we are in politics to serve. I want to see a country where the weak and the vulnerable don’t suffer, but I also want us to be a party that represents the majority of the country.

“I am a loyal person: I am never going to leave the party I love, but Jeremy’s politics isn’t right for Labour or the country so I wouldn’t serve in his shadow cabinet if he were to be elected.”

She rejects the idea that the Labour Party could split, saying: “That’s not what I want, and that’s not what I believe will happen. I can no more leave the Labour Party than leave my own family. And you never stop fighting for what you believe in. I won’t quit. I won’t quit this race and I’ll never leave the Labour Party.”