Labour is to ask the civil service to begin working up proposals for a fresh referendum on Brexit because of the likelihood of a general election later this year, John McDonnell has said.

The shadow chancellor said Jeremy Corbyn would seek formal discussions with civil servants when he meets Sir Mark Sedwill, the cabinet secretary, on Tuesday.

The pair will meet to discuss Labour’s calls for an independent investigation into reports that senior civil servants had claimed the Labour leader was “losing his memory” and “too frail”

Mr McDonnell said Mr Corbyn would also Sir Mark for permission to open talks with civil servants on Labour’s Brexit plan, including proposals for a Final Say vote.

Speaking at a briefing in Westminster, he said: “At the moment the permanent secretary and cabinet secretary have said no to any pre-election discussions or meetings with the civil service. That was a civil service decision but they did consult Theresa May, and Theresa May blocked those meetings as well.

“This morning, because Jeremy is meeting Sedwill about this issue of the inquiry, I’ve asked him to raise at that meeting that we should now have access to the civil servants because of the likelihood of a change in prime minister happening and the likelihood therefore of a general election in the autumn. I think that’s just the pragmatic and practical thing to do.”

Asked if the potential discussions would involve civil servants being asked to draw up plans for another referendum, he said: “At the moment we’re consulting on our attitude in a second referendum but Jeremy has made it clear that it should go back to the people in a referendum.

“We’d expect, just as we’d expect the civil service to prepare for an incoming government, for them to prepare for all options, so that would have to be an option they’d have to prepare for.”

Mr McDonnell said he had previously warned senior civil servants not to repeat the “mistake” made by David Cameron in the run-up to the 2016 referendum, when the then-prime minister told the civil service not to prepare for the possibility that Leave would win.

He said: “We’re getting ourselves into that situation at the moment where they’re not capable, because of that lack of dialogue with us, of preparing for a change of government. So it’s only responsible to enable those meetings to take place.”

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The shadow chancellor also insisted that he had no fears about discussing Labour’s policies with civil servants, despite the row over the reported comments about Mr Corbyn’s health.

He said: “I believe in the independence of the civil servants. From Northcote [the civil service reforms made in 1854] onwards that tradition has served us pretty well. Unfortunately this incident has demonstrated that obviously there’s a few people there that haven’t behaved to the standards that we would expect of the British civil service.

“I’m hoping that will be resolved quickly and we can start having these meetings, informing the civil service of the development of our policies and letting them prepare them for the incoming Labour government.”

Mr McDonnell said he believed that the “instability” of Boris Johnson made a general election later this year more likely.

He said: “I think it’s almost impossible to tell what will happen in September and October.