On the Thursday before Easter, Westminster journalists were invited to bring their children to No 10 Downing Street for an Easter egg hunt. Gold chocolate eggs of all sizes were hidden in the flowerbeds. There was much excitement as the young guests tore around the lawns in pursuit of the hidden treasure. But for the parents, the occasion lacked one vital element. “There were loads of eggs, just no prime minister,” remarked one. Officials explained that the PM was “otherwise engaged” and unfortunately could not be there.

There had been signals earlier in the same week that, with hindsight, should have invited suspicion that Theresa May was up to something. Why, with Easter approaching, was there was there still no date in the political calendar for the Queen’s speech, normally held in May? The education secretary, Justine Greening, had spoken of the prime minister’s passion for grammar schools days before, but it was strange that no one would say whether there would be legislation to create more of them in the next parliamentary session.

And why had a long-rumoured announcement on capping energy prices been held back yet again, when it was bound to be hugely popular? An impression of indecision was forming around the May administration as she secretly plotted the most startling move and most brazen U-turn of her premiership so far – the decision to call a snap general election when she had all but promised not to do so.

The prime minister told only a handful of her Downing Street inner circle before last Tuesday’s announcement. One of her closest aides had been informed a week before – but was not told when the announcement would be made, or when the election would take place. May is known to keep her cards close to her chest, but this was a tightly held secret even by her standards.

The Tory party chairman, Patrick McLoughlin, who had been urging the PM to go for an early election for weeks, was kept in the loop. But for the rest of Westminster – Conservative ministers and MPs included – the news was a bombshell.

General election campaigns are normally months, even years, in the planning. But when May announced that voters would go to the polls on 8 June 2017, not May 2020, all the parties had to drop everything in an instant. Manifestos that had hardly been thought about had to be finalised within a fortnight, while campaign teams had to be set up from a standing start and parliamentary candidates selected in hundreds of seats within days. In addition, funding had to be organised and donations sought, literature printed, slogans invented, battle buses ordered and painted, and all other priorities shelved.

'The walks give clarity': how Wales hike helped PM decide on next step Read more

“We had all thought the moment had passed,” said one senior Labour official. “There had been some minimal planning early in the year when we thought she just might possibly call an election on the same day as the local elections, but when that window closed, nothing more was done. When this kind of thing hits you, you just start running in any old direction to try to catch up and it is often the wrong direction.”

The shock was all the greater because May had earnestly insisted so many times that now was “not the time” to go to the country, with Brexit – the biggest peacetime challenge for a government in decades – on her plate.

There was also the Fixed-term Parliaments Act, passed with May’s backing in 2010, which decreed that elections should be held every five years and not be called according to the political convenience of the governing party. May worried little about that. Her decision meant that everyone from party leaders to backroom staff had to appear eager and ready for battle. But many were neither. “It is not just logistically difficult to go from nowhere into a campaign. But it is tough on lots of people, those with families and children,” said one senior opposition party source. “The parties have to compensate staff for holidays they have to cancel. It costs far more to print posters and election material at short notice. And you have to arrange events and launches and find the people, the human props, to be at them, holding placards, with a few days’ notice.”

Within hours of the announcement, Labour summoned a meeting of the officers of its national executive committee. They discussed how to rush through selections of parliamentary candidates under an accelerated process. Normal procedures involving hustings in local parties, followed by votes in the constituency Labour parties (CLPs), would have to be ditched and candidates appointed directly by the NEC. The next day, the full NEC met to thrash out an outline election strategy and a slogan. “We came up with: ‘The many, not the few’, which, somewhat ironically, was pretty much what Blair had in 1997,” said one who was there. “It worked for Blair, so maybe it can work for Jeremy.”

The MPs Andrew Gwynne and Ian Lavery, named previously as campaign co-ordinators, had to dust down sketchy election plans drawn up months before when no one really believed there would be a poll, and started thinking how to put flesh on the bones. Tom Baldwin, who was Ed Miliband’s press secretary for the 2015 election, says that, inevitably, Labour will be terribly underprepared. “General election campaigns are a highly complex, arduous and testing undertaking even at the best of times. And for Labour, it is pretty obvious these are not the best of times. The party is unprepared for a snap election, with basic campaign components like a list of key seats or a manifesto not yet oven-ready.”

The Tories have to work just as fast. Senior Conservative MPs say they will use rules normally used to appoint candidates for byelections to fill their list in haste.

The Lib Dems had selected 325 of their candidates a year ago, but are also some way short of being completely battle-ready. Party leader Tim Farron called a series of meetings from Tuesday. The party’s slogan, “Open, Tolerant, United”, was confirmed. It was decided that the party would not enter a coalition with either the Tories or Labour, positioning it as the main opposition party in waiting, and the only one able to challenge May over Brexit. Nick Clegg, who took the Lib Dems into coalition with the Tories in 2010, is fine with that, says the party. New members were urged to join – 8,000 have done so in past few days – taking the total from 45,000 in 2015 to 95,000 now, all paying a minimum of £12. An email was sent out asking for donations, which raised £500,000. The party insists it will not be short of money, even if it is short on time.

Labour’s problems run far deeper. 'It is not the lack of time. It is that we are not a team,' said one senior figure

Labour, with more than 500,000 members and its balance sheet healthier than it has been for decades, is also confident it will have enough money. Its problems lie elsewhere. There is virtually no one in Jeremy Corbyn’s inner circle with experience of running a general election campaign according to normal timescales, let alone this one. But Labour’s problems run far deeper. “It is not the lack of time. It is that we are not a team,” said one senior figure.

Less than a year ago 172 Labour MPs (more than three-quarters of the total) voted in favour of a motion of no confidence in Corbyn. They now have to decide how to campaign on the doorsteps when they are known to think their leader is not up to the job. One Labour MP in a northern seat said she had no option but to fight just on local issues. If asked about Corbyn she would say she still did not think he was a potential prime minister. Asked whether his picture would appear on her campaign literature, she said: “I have not had time to order any. But no, it won’t. You can be sure, though, that it will be on every single piece of paper the Tories and Lib Dems put through letterboxes up and down the country.”

She is not alone in taking that view. Theresa May may have lost some of her reputation for consistency, but she has won begrudging plaudits, even from opponents, for sheer political opportunism. “She has played a complete blinder,” said one Labour strategist. “She has called an election when she is still on her political honeymoon and we are split down the middle. It is completely cynical but politically brilliant, even more so as no one expected her to do it.”