Jeremy Corbyn is the overwhelming favourite to win the Labour leadership contest, according to the latest Opinium/Observer poll, which shows he has more than twice the level of support among party supporters as his challenger, Owen Smith.

Owen Smith: who is the man challenging Corbyn as Labour leader? Read more

The online poll finds that among those who say they back Labour, 54% support Corbyn against just 22% who would prefer Smith. Some 20% say they are undecided and 4% say they do not intend to vote.

The findings offer further proof that grassroots support for Corbyn remains firm despite weeks of bitter conflict between the leader and Labour MPs at Westminster, a large majority of whom backed a vote of no confidence in him after the EU referendum.

While supporters of Smith, the former shadow work and pension secretary, will argue that he will gain support as he becomes better known in the run-up to the 24 September vote, it would appear he has a mountain to climb.

Some 54% of Labour supporters say they approve of Corbyn’s leadership after almost a year in the job, while 24% say they disapprove. The remainder had no fixed view.

Labour signed up more than 183,000 new registered supporters in 48 hours last week, all of whom will now be eligible to vote having paid the £25 fee imposed by the party’s national executive. But it is not known how many are supporters of Corbyn and how many joined in order to vote to oust him.

On Saturday, Corbyn said the party’s huge membership, which now totals over 500,000, would power it to victory at the next general election in 2020 under his leadership.

Opinium found, however, that while Corbyn is popular within his own party in the country, his ratings are catastrophic when voters are asked about his suitability to be prime minister.

When all voters were asked who they thought would be the best prime minister – Corbyn or Theresa May – 52% chose May and just 16% Corbyn. Among Labour supporters 48% said Corbyn and 22% May, while among Tory supporters 88% said May and 4% Corbyn.

Support for the Conservatives – despite their splits over Brexit – is at 37%, six points ahead of Labour on 31%, with Ukip on 15%, the SNP 6% and the Greens on 4%.

Opinium Research carried out an online survey of 2,231 UK adults from 19 to 22 July. Results have been weighted to nationally representative criteria.