Jeremy Corbyn’s strong performance during the election campaign must not disguise the fact that the party lost a contest that was “easily winnable”, a former senior aide to Tony Blair has warned.

Two more weeks and Labour would have been in power, says shadow chancellor Read more

Peter Hyman, the former prime minister’s head of strategic communications, said that Corbyn deserved significant credit for an effective campaign and an ability to energise young voters.

However, he told the Observer that this success should not hide the fact that the party fell to a third successive loss and had been boosted by an “incredibly strong protest vote”.

“Of course Jeremy Corbyn did better than anyone expected in this election, including many in the Labour party,” he said. “He deserves credit for running a far better campaign than Theresa May, but there has also got to be some realism about this. Labour has lost a third election in a row and it was an election that, with the right people and programme, was easily winnable.

“If you think the Tory majority was only [small] and May fought a terrible campaign, this should have been an election that Labour won. It is like a football match where you are 5-0 down, you claw it back to 5-4 and claim you won the cup. Well we haven’t won the cup. We have done better than expected, but we are not in power. None of the policies that people want from that manifesto are going to be implemented.”

Extensive plans were drawn up before the unexpectedly strong performance for a comprehensive, two-year review of the party’s purpose and policy platform. Moderates now fear that Corbyn’s strong showing, which saw him secure the party’s highest vote share since 2001, will disguise Labour’s shortcomings.

Many moderates spoke out in the immediate aftermath of the election result to praise Corbyn, with Lord Mandelson among those to say the Labour leader had managed to deliver a political earthquake. Senior figures, including Chuka Umunna and Angela Eagle, are now understood to be happy to return to the frontbench.

Facebook Twitter Pinterest Chuka Umunna and Angela Eagle, both former opponents of Jeremy Corbyn, are believed to be happy to return to Labour’s frontbench. Photograph: David Levene/The Guardian

However, others in the party are starting to break cover to suggest that Corbyn narrowly failed to defeat a Conservative party that ran one of the worst campaigns in British political history. Writing in the Observer, Jess Phillips, the MP for Birmingham Yardley who has been critical of Corbyn, says that opponents of the leader will now divide into three groups. Some would “jump on the team Corbyn bandwagon”, while some will “remain a thorn in his side”.

“The group I will be in is the one that recognises that we didn’t win this election but we are on the way to winning the next one if we get it right,” she writes. “I will never be a blinkered cheerleader of anyone (ask my kids) and while I got his electability half wrong I would be doing him and the country a disservice by donning the white robes of worship and ignoring my concerns.”

Hyman accepted that there was an appetite among the public for less austerity and funded investment in public services, but urged the party to now try to combine “the idealism and fresh politics which Corbyn tapped into with a proper programme for government”.

“We’ve still got a huge mountain to climb,” he said. “Credit to Corbyn in terms of fighting a better campaign. Credit in terms of energising young people. But it all has to be coupled with a genuine realism – we have lost.

“The sort of ‘one more heave’ approach – more of the same next time will push us over the line – would be completely misguided. Spending more money is not the whole answer to lots of problems. It has got to be money and fresh thinking. If we want to improve education in the university sector, it is obviously popular to say we will get rid of tuition fees, but that is not a programme – that is not a policy prospectus. It is a single piece of spending.

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“We have to see this for what it was – an incredibly strong protest vote for Labour. It is not yet the public saying they want Labour in power. We’ve got to work out seriously and not be carried away with the result.”

He spoke after Chris Leslie, the former shadow chancellor and critic of Corbyn, said that Labour missed an “open goal” by losing the election. He said Labour had secured an “OK result” by increasing its share of the vote and securing 262 seats.

He told the BBC Radio 4’s Today programme: “We haven’t won that election. We shouldn’t pretend that this is a famous victory. It is good as far as it’s gone but it’s not going to be good enough. I’ve never known a more beatable prime minister than Theresa May – brittle, I think. Very, very wobbly and shaky indeed.”