Jeremy Corbyn would have become prime minister at the head of a majority Labour government if the election campaign had gone on for two more weeks, the shadow chancellor, John McDonnell, claims today.

Writing in the Observer, after Labour confounded its critics and most of its own MPs by depriving Theresa May of an overall majority while increasing its Commons numbers by 32, McDonnell says political momentum was firmly behind his party because the British public had recognised Corbyn’s decency, strength and principles. This, he argues, was despite a barrage of “poisonous Tory attacks” and a “policy-free”, expensively funded Tory campaign, cheered on and applauded by the dominant rightwing elements of the press.

Although many Labour MPs refused to mention Corbyn on their campaign literature, believing him to be an electoral liability who voters could not envisage in Downing Street, the Labour leader can expect a thunderous, hero’s reception at a parliamentary Labour party meeting in the Commons on Tuesday evening. The same is expected at prime minister’s questions when he will face May across the dispatch box on Wednesday.

Senior Labour sources said Corbyn was likely to carry out a reshuffle of his shadow cabinet and frontbench teams, amid speculation that he will open the door to a number of leading figures who had previously resigned or refused to serve in his shadow administration. It is expected offers will be made to the likes of former shadow business secretary Chuka Umunna, and former cabinet minister and shadow home secretary Yvette Cooper, both of whom had been preparing potential leadership campaigns if the election result had been bad for Labour and Corbyn had quit.

The future of Diane Abbott, the former shadow home secretary, who pulled out of the latter stages of the campaign because of illness, will also be decided. While she is expected to remain out of frontline action for some time, it is expected that the way will be left open for her to return to the shadow cabinet when her health improves.

In his article, McDonnell says he had always expected that the Tories’ 20-point lead in the polls when the campaign began would narrow once voters were exposed to the Labour message and the Corbyn campaign. He argues that Corbyn and Labour benefited from fairer media coverage once election broadcast rules were enforced: “I argued that when the broadcast media rules kicked in during the election period, we would have a better chance to get our policies across. The more balanced the broadcast coverage of Jeremy Corbyn, the more people would see him for the honest, decent, principled, and indeed strong leader he was.”

John McDonnell Photograph: James Gourley/REX/Shutterstock/Rex

Victory would have been Labour’s with more time, says McDonnell. “My judgment is that if the campaign had been a couple of weeks longer we would have secured a majority, given the narrowness of the voting in so many seats.”

Sources close to Corbyn said the party would in the coming weeks be using all parliamentary devices available to promote the same economic and social policies it advanced during the campaign, and in its manifesto. “We will be saying that, while we did not win a majority, neither did the Tories and that there was a huge level of support for what we’re saying. We will be promoting those ideas all the way with an eye on the future and in the full expectation that this government will not last the full course.”

McDonnell says: “We will place before parliament policies drawn from our manifesto that we believe are needed to address the challenges Britain now faces and can command support.” Labour is making preparations for an alternative Queen’s speech to challenge May’s.

Labour will also argue that the Tories now have no mandate for their own programme, including May’s plans to expand the number of grammar schools. The message will be that, having connected with the electorate, the party is preparing for a possible second general election, whenever it may come.

McDonnell says May called an election because she needed a mandate for Brexit negotiations; ending up with no overall majority meant she has no authority to drive through her version of Brexit.

“She told us … that she specifically called the election to secure a clear mandate … to negotiate a Tory Brexit. She failed. It is absolutely clear that there is no majority for the race-to-the-bottom Brexit backed by Theresa May, and that an alternative is required reflecting the common ground that appears to have emerged in this debate. This is the implementation of a Brexit that respects the referendum decision and secures the greater freedoms leaving the EU achieves, but which best protects our economy, jobs and living standards.”