Labour leadership challenger Owen Smith has admitted his campaign to dethrone Jeremy Corbyn has been "long and bruising' in an open letter to party members on the eve of ballots closing tomorrow (20 September).

The former shadow work and pensions secretary warned that Labour was "at a crossroads" between reuniting the party or "satisfying ourselves with ongoing division and opposition".

"It's the clearest choice we have had to face in a generation. A choice that will not just determine the future of our party, but the future of the millions of people in Britain who need Labour in power," he added.

Smith, who was little known outside of Westminster before the election, said he was "proud" to bring forward a "positive programme" of policies, including a pledge to introduce a £200m investment programme.

But Smith is expected to lose the election to Corbyn, who attracted almost 60% of the vote in 2015.

The bookmakers have the Labour leader as favourite, and he has attracted the most Constituency Labour Party nominations and endorsements from affiliated trade unions.

The latest opinion poll from YouGov for The Times, of more than 1,200 people between 25 and 29 August, put Corbyn 24 points ahead of Smith (62% versus 38%).

A defiant Smith, however, told IBTimes UK on Friday: "Don't write me off until the final whistle, is all that I ask." The "final whistle" will be around midday on Saturday when the winner of the leadership contest is announced at Labour's annual conference in Liverpool.

Owen Smith's open letter to Labour members and supporters

Dear friends,

As the ballots close and this leadership contest draws to an end, I wanted to take this opportunity to speak directly to Labour members and supporters.

This has been a long and bruising contest, and I know many of you didn't want it to happen. But the truth is it had to happen.

Our party is at a crossroads, and the choice we face is between renewing our party to pursue unity and power, or satisfying ourselves with ongoing division and opposition.

It's the clearest choice we have had to face in a generation. A choice that will not just determine the future of our party, but the future of the millions of people in Britain who need Labour in power.

I entered this contest because I want to see a Labour government again. I want to see a radical, transformative government that can change our country for the better and bring an end to another generation of Tory rule.

I saw what the Tories did to our country in the 1980s and I see what they are doing to it again: workers' rights being stripped away, millions of patients trapped on NHS waiting lists, and the reintroduction of grammar schools.

It's a hard right agenda, which could do untold damage to our country; and unless we have a radical, credible opposition to the Tories then we won't be able to stop them – now or at the next election. That's the straight talking, honest truth. I regret the state we are in but I don't regret being the one to say it.

Throughout this campaign I have been deeply humbled by the support and backing I have received from people and organisations across our movement: GMB, Usdaw, Community Union, the Musicians' Union, the Socialist Health Association, Jewish Labour Movement, Labour Movement for Europe, hundreds of volunteers and more than 1,000 councillors.

Together we have had more than 300,000 conversations with Labour members and supporters, and held more than 100 events in towns and cities across Britain: from Glasgow to Gillingham, Cambridge to Cardiff, London to Loughborough. Throughout, I've been inspired by Labour members in every corner of the country.

Many of you have dedicated a huge part of your lives to our party, because you know how vital it is that we are able to serve the people who desperately need us in government.

I'm proud that I have brought forward a positive programme for a future Labour government that would improve the lives and livelihoods of millions of our fellow citizens: a £200 billion investment programme to replace austerity with prosperity; a revolution in workers' rights; an extra £60 billion for our NHS paid for through new taxes on the wealthiest; and trusting the British people to sign off on the final Brexit deal.

These ideas will remain as relevant after this contest as they have been during this contest.

They are part of my vision for Labour and Britain's future and whatever the outcome of this contest I will continue to make these arguments and do all I can to see us back in government.