Speaking to the Yorkshire Post on the final day of party conference, Tom Watson admitted there were concerns that June's manifesto was “only relevant in certain areas” and the party needs to “listen” to voters.
He also became the latest Shadow Cabinet member to refuse to rule out a second referendum on Brexit, after London Mayor Sadiq Khan suggested it was "possible" the next manifesto could include a vote on the final deal.
He told regional reporters a referendum was "highly unlikely" but stressed the party is "not ruling anything out".
His comments come at the end of a four-day conference which has been dogged by reports of fresh tensions between hardline "Corbynistas" and the more traditional Labour members.
Behind the scenes, there are still concerns that Jeremy Corbyn does not command the broad appeal the party would need to win the next election.
Pressed on these divisions, Mr Watson insisted that the June election marked a turning point in Mr Corbyn's leadership and the atmosphere within the party.
"Because Jeremy did so well at the election - because he's got renewed authority, because he's grown into leadership - he's pulled everyone back around," he said.
"We've come out of [the election] with the party more united than ever before and bigger than its been in my lifetime."
The party has already drawn up a list of just over 70 target seats it is hoping to win at the next election, including Shipley, Pudsey, Morley and Outwood and Whitby and Scarborough.
In all of these seats, the incumbent Tory candidate managed to increase their vote share despite the surge in Labour support.
Asked how Labour plans to win over those voters who were turned off by the party's message in June, Mr Watson acknowledged there were some concerns that the manifesto was "only relevant in certain areas"
But he said the party has now got to "focus on listening to voters in those Yorkshire seats" and "convince them we are going to run the economy well".
One of the issues that has dominated this year's conference are the party's rifts over Brexit policy.
On Sunday a group of more than 30 Labour MPs and peers wrote to the Observer calling on the party to commit to keeping the UK in the Single Market. And in an interview with the Evening Standard on Tuesday, London Mayor Sadiq Khan suggested a second referendum on the final withdrawal deal with the EU could make it into the party's next manifesto.
A number of frontbenchers have failed to rule out the possibility of a referendum this week, including shadow communities secretary Andrew Gwynne.
Commenting, Mr Watson told reporters that the party is "not ruling anything out" – but he went on to predict a second referendum was “highly unlikely”.
"We want to exit from the EU, that's the decision from the referendum... We're not ruling [another vote] out but it's highly unlikely,” he said.
He also cast doubt on the prospect of the UK remaining a member of the single market "after the transitional period".
He said: "Our negotiating position is to have tariff free access to the European market. And I think that is the likely outcome.
"I think it's highly unlikely that a deal will encompass membership of the single market after that transitional period."