The Labour party could take another two years to rebuild its credibility with the electorate, the shadow chancellor as said.
John McDonnell told the i newspaper that the media had not given his party a “fair hearing” but that it would get its message over by “word of mouth”.
He blamed the economic crash of 2007 for the party’s woes and said he had been punished for being in power at the time.
“The reality is that Labour was in power in 2007, 2008 when the crash occurred,” he said.
“Right across Europe, whoever was in power then was blamed for that economic crash, and Labour has never recovered.
“We have been consistently about 20 points behind in economic credibility in the polls since then.”
Mr McDonnell, who is a longtime ally of party leader Jeremy Corbyn, admitted that the party had made no progress in turning around voters’ view of it since the general election.
“I think it will turn, but it’s obviously hard work, of course it is.
“Over the next 18 months, 24 months, I think it will turn and we will be ready for the general election at that stage.”
His analysis appeared to have become more pessimistic than the one he gave The Independent a month ago, when he said that the party would be turned around “over the next 12 months”.
Mr McDonnell last month warned of a “soft coup” taking place against Mr Corbyn’s leadership, which he said was being orchestrated by elements in the Murdoch press and the Parliamentary Labour Party.
He later withdrew the comments, with a spokesperson saying that they were no longer his view.
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