The Conservatives were “simply not ready” to fight a shock election when Rishi Sunak sent the country to the polls, according to a major investigation into the Tory disaster.

Number 10 were unaware of the lack of readiness and party headquarters looked caught on the hop, the report by new campaigning group Conservatives Together claims.

The report – What Went Wrong – will be launched at the Tory conference this week as the party struggles to come to terms with the scale of the loss.

Writing in the foreword, former party chairman Grant Shapps and Lord Kempsell state: “For future generations of Conservatives, 2024 will be a year that causes shudders down the spine. Five years after our biggest majority in decades, it seems that our entire vote-winning machine collapsed.”

But the report argues that if one per cent more of the electorate had voted for the Conservatives they would be the biggest party in Parliament. The support of an extra two per cent would have resulted in a “resounding victory” with them winning 345 seats instead of 121.

What Went Wrong details the chaos that followed Mr Sunak’s announcement of a snap election, stating: “CCHQ seems to have been caught unawares by the announcement. The party was simply not ready to fight an election, but No10 seems also to have been unaware of this.”

A candidate told the authors: “We simply didn’t have the time or resources to run effective campaigns. What plans we had to leaflet, canvass and fundraise over the summer had to be abandoned.”

The report also highlights the pressing challenge of finding hundreds of candidates to fight the next Westminster election: “We now have over 500 seats to fill. That presents a massive opportunity for renewing our party’s ranks, but also a daunting challenge.”

The four party leadership candidates will set out this week in Birmingham how they would lead the Tories back into power.

The report notes the challenge posed by parties other than Labour, stating: “The rise of Reform UK and resurgence of the Liberal Democrats pose enduring challenges to the traditional Conservative coalition. These third parties – not Labour – have absorbed the majority of dissatisfied Tory support.”

However, the authors believe there is a “natural conservative majority that exists in the United Kingdom”.

Conservatives Together is clear about its goals.

Mr Shapps and Lord Kempsell write: “We are not a think tank. Conservatives Together will focus almost entirely on campaigns and communications. It is also not affiliated with one or other wing of the party, or one or other leadership candidate.

“Whoever leads our party can expect to benefit from Conservatives Together’s work.”

But Labour’s Pat McFadden, a key advisor to Sir Keir Starmer, said none of the candidates showed any sign of appreciating the scale of change required.

Comparing the state of the Tories to Labour after Jeremy Corbyn’s second defeat, he said: “It’s shocking that after such a terrible election result there is so little appetite for change in the Tory leadership election. They seem to think if they shout the same message louder the voters will admit they were wrong. And they are so terrified of Reform that they have abandoned all claim to the centre ground.

“Labour learned some hard lessons from defeat. It looks like the Tories have learned nothing at all.”

Tory party chairman Richard Fuller will today make a “profound” apology to the nation for the election disaster which resulted in a “reckless, ideological socialist Government” taking power.

He will kick off the conference in Birmingham, saying he is “deeply sorry” for Labour winning a “huge majority based on a paltry share of the electorate”.

Mr Fuller will also vow to take on Nigel Farage’s Reform UK which won more than four million votes and took five seats.

Acknowledging mistakes in how Tories faced up to this electoral threat, he will say: “Well, we gave them oxygen. We gave them space.

“We will take both back.”