Rishi Sunak’s top aides advised him not to call an early election, warning him that voters would be less likely to feel “financially optimistic” in the summer and that Conservatives would not be able to “hit Labour hard with both fists”.

Isaac Levido, who directed the election campaign for the Conservatives, and Michael Brooks, a Tory strategist, issued the warning to the former prime minister in a blunt memo on 3 April, seven weeks before the election was called.

The pair strongly argued that Sunak should delay the election until after the summer. “It is strategically most beneficial to have an autumn election in October or November,” they explained in the memo, revealed in The Sunday Times.

“We need as much time as possible for economic metrics to improve and for voters to feel better off. An earlier election gives us less scope to communicate about economic progress, because voters are less likely to feel financially optimistic.”

Calling an election before the summer would remove “potential positive psychological effects of summer”, including those resulting from lower energy bills, holidays, better weather, the Euro 24 football tournament and even the Olympics, according to the memo.

The memo acknowledged there was a risk for the Conservatives that a late election “could leave us vulnerable to internal party division and other off-message distractions and policy challenges (eg strikes, increased Channel crossings)”.

Sunak and his aides had, by then, given up hoping that the Bank of England would successively cut interest rates. But, the memo argued, going to the country early would mean the Tories would have to communicate more “wedge” issue policies “because we would have less ammunition to fight on the economy”.

The memo concluded: “The election will be a fist fight, and we want to be able to throw punches with both fists – our economy fist, and our policy platform/reform fist … in summer, our ability to fight on the economy will be weaker, meaning we will have to punch harder with our reform fist in order to hurt Labour and inject urgency into the campaign. Whereas in autumn, our ability to throw punches on the economy will be stronger, meaning we can hit Labour hard with both fists.”

The memo was unveiled in The Times in extracts from a new book, Out: How Brexit Got Done and the Tories were Undone, by Tim Shipman.