Labour has a mountain to climb to win the next election outright, and is still failing to chalk up big enough leads on image or leadership to make it likely to secure an overall majority, according to polling which will be put to a Labour conference to be addressed this weekend by Ed Miliband.

The YouGov polling, commissioned by Progress, suggests the party is still seen as "nice" but incapable of taking tough decisions. Miliband's personal ratings have hardly improved over the past year.

In an article for Progress, the New Labour pressure group, the YouGov president, Peter Kellner, describes the polling as "profoundly troubling" for Labour, saying that despite the unpopularity of the government, Labour has uncomfortably small leads and has been unable to generate wide public enthusiasm.

He writes: "The central fact is that no successful opposition in the past 50 years has gone on to regain power with such a weak image and without achieving much bigger voting-intention leads at some point in the parliament."

Labour, he advises, needs to think what it will do if it fails to win an overall majority.

Kellner points out there has not been a single time in more than 80 years when an opposition has returned to power at the first attempt with an overall majority. He also suggests no opposition has gone on to win power without at some point achieving a lead of at least 20%.

The biggest Labour lead recorded by any opinion poll during its current period of opposition was 16%, recorded by TNS last September. The latest YouGov poll gives Labour a lead of 11%.

Kellner suggests that Miliband may be holding back party support. When voters are asked if they would prefer a Labour government led by Ed Miliband or a Conservative government led by David Cameron, the two options are level pegging: Labour 41%, Conservative 40%.

The polling also shows that by a margin of 50-35 points, voters regard Labour as "nice" – but by a larger, 61-24, margin, also as "dim". Most people consider the Tories both "mean" and "dim"; but more people regard the Tories as "smart" than say the same about Labour.

Labour is ahead of the Tories on many of 12 key attributes, but Kellner says on many the lead is "uncomfortably small – the economy, learning from past mistakes and having people who are up to running the job".

He adds: "No opposition could be happy with the fact that, when the economy is flatlining, just one person in three thinks it [the Labour party] would take the right decisions to secure greater prosperity.

"On just two measures do the optimists about Labour's prospective performance outnumber the pessimists, and then by only a modest five points: being 'on the side of people like you' (43-38) and delivering good value services (41-36)."

He points out: "There is one issue on which the Conservatives hold a large lead, and it could be decisive in a tough election campaign. By two to one, voters think the Tories have the courage to take tough and unpopular decisions. By three to two, voters think Labour lacks that courage." Kellner adds: "For a party whose greatest campaigning challenge is to appear reassuring, this should be profoundly troubling."

If Labour/Conservatives win the next election, do you think that, in general, they will or will not...

He also highlights other YouGov polling showing that only 36% of those surveyed credited Miliband with any positive attributes. More than two years later, that figure has crept up to 41%.

Assessing the number of Tory seats Labour needs to win to gain an overall majority, Kellner says he does not expect the party to be able to take more than between eight and 14 Liberal Democrat seats. As a result, he suggests it will need to take about 60 Tory seats to secure 326, the minimum needed for an overall majority.

Within England, the key marginals are fairly evenly spread: 17 in the north, 16 in the Midlands, and 23 in the south. In national terms, Kellner says, this means a five-point swing to Labour converting the Conservatives' 7% lead in 2010 to a Labour lead of 3% in 2015.

But he says the swing may need to be higher due to the bonus normally enjoyed by sitting MPs standing for re-election for the first time. Fifty-seven of the 60 Labour targets are seats held by the Tories with the MP standing for re-election in 2015. He estimates that bonus can be worth 1,000 to 2,000 votes. That in turn increases the swing that Labour requires.

Kellner also points to a rare stability between the left's and the right's share of the vote over the past two years. It has remained at 54% to 40% over that period even if the Ukip share on the right has increased at expense of the Conservatives, and Labour has risen at the Liberal Democrats' expense. "The stability, in net terms, of the overall left-right division is striking. It contrasts with other midterms that have served as a prelude to a change in government."