Speculation has been rife for months about when the prime minister will choose to go to the polls, with some pundits believing he would call one in May to coincide with the local elections.

UK general elections have to be held no more than five years apart, so the next one must take place by 28 January 2025 at the latest.

This is five years from the day the current parliament first met (17 December 2019), plus the time required to run an election campaign.

The phrase "working assumption" does give Mr Sunak wriggle room should circumstances change, and he has not ruled out a spring election.

Sky News spoke to pollsters about the factors the prime minister will be weighing up in making his decision - and when they think the election should be.

"I am absolutely clear it will and should be the autumn," Conservative peer and pollster Lord Robert Hayward told us.

He said the National Insurance tax cut announced by Chancellor Jeremy Hunt in November will "take time to filter through", as will the "perception inflation is really on its way down".

He added that the Conservatives are "still carrying the burden of the events of 2020 to 2022, and they need to put them as far away as possible".

Lord Hayward predicts the Tory party conference in October will be the "launchpad for the election", meaning voters will be casting their ballots "probably on 14 November or around that date".

With a US election set for 5 November, that would mean the campaigns on each side of the Atlantic colliding - a scenario that has not happened in decades.

Officials in Whitehall are said to have warned Downing Street against this because of "security risks".

However, Lord Hayward said while the US election may be of "slight concern" he doesn't believe it will be a "deciding factor" in when we go to the polls.

'Mid-November election likely'

Polling expert Professor Sir John Curtice comes to a similar conclusion.

With the Tories trailing Labour by around 20 points in the polls, he believes that "frankly, they are heading for crucifixion".

He said that, despite the noise from Conservative MPs about immigration, the economy "is the most important issue for voters", followed by the NHS, and the government needs time to make progress in these areas.

Like Lord Hayward, he believes Mr Sunak may fire the starting gun for the election in his speech to close the Conservative Party conference in Birmingham on 2 October, which "could mean an election on 14 November".

He said all parties are holding their conferences earlier than usual this autumn, with the Tory one happening last - perhaps giving an insight into the prime minister's thinking.

Sir John was never convinced by the May election rumours - saying it is unlikely Mr Sunak "would risk" cutting a two-year term to 18 months for an election he is expected to lose.

"At the moment, there is no good reason for them to do anything other than play it long."

Cross over with US election 'could compromise media coverage'

However, Sky's election analyst Professor Michael Thrasher has predicted a slightly earlier date of late September or early October.

He said while some are tipping mid-November he is "not so sure about this scenario" because of the overlap of the US election "meaning that media coverage of our own election would be compromised".

He said media coverage will be essential to the Tories' campaign strategy, which is "likely to focus on Labour's manifesto and the likely absence of detailed policies regarding economic growth, management of the NHS, housebuilding and last but by no means least, the migration crisis".

"That suggests late September or early October instead", he added.

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Spring election 'could still be possible'

Prof Thrasher said that June through August "can be ruled out" because the Tories are likely to do badly in the May local elections "so why would they willingly go back so soon for another beating?".

But while a May election is looking less likely, he says this is "not impossible".

The spring budget, scheduled for earlier than usual on 6 March, is likely to "contain headlines designed to persuade voters that the Conservatives are, at heart, the party of low tax, despite all evidence to the contrary" and may prove a good time to go to the polls if well received.

That could mean a general election to coincide with the local elections and mayoral elections on 2 May - a date which would also "give the Conservatives the opportunity to exploit hostility towards taxes on motorists in London and elsewhere", following the backlash to Sadiq Kahn's ULEZ charge.

Spring election 'could minimise Tory losses'

Despite Mr Sunak's resistance to the idea, some strategists believe it may be in the Tory party's best interests to go early in order to stem losses.

Lord Daniel Finkelstein, a former adviser to Sir John Major, warned there are costs of holding onto power.

"When I look back on the 1997 election, I think one thing we could have done to mitigate the size of our defeat is to have gone slightly earlier," he told Sky News.

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Lord Finkelstein said while he can "understand the temptation" for Mr Sunak to wait it out in the hope of turning things around, that "serendipitous occasion" may not occur and things could even get worse.

He pointed to potentially bruising local election results in May and the fact that Channel crossings are likely to rise over the summer, while the mortgage crisis may deepen as more people face the end of their current fixed rates.

This would be damaging going into an election where opposition parties will be making the case for change, and the Tories' best bet is to argue "the country is on the right track, and we are turning things around".

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"It's very hard for any prime minister to call an election which they are quite likely to lose. While the temptation to go on will be strong, putting it off will make things more difficult if more problems arise," Lord Finkelstein said.

"The timing of the election will not be the predeterminer of the outcome. It will be the fact that Boris Johnson and Liz Truss let down the country and it will be very difficult to turn that around."

'Spring election rumours keeping Labour on their toes'

Mr Sunak's election indication came just days after shadow frontbencher Emily Thornberry claimed a May election is "Westminster's best kept secret".

Labour has been preparing for office for some time and has factored in the possibility of a spring election.

"Our job is to be ready whenever it comes, and we will be," said one Labour source.

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However, Sir John believes the leaks of a spring election were designed purely "to keep the opposition on their toes... creating uncertainty around campaign plans and policy announcements".

"If the Labour lead is halved to eight or nine points, then there may be an argument to say 'let's go early, we might lose, but we will keep some seats, there could even be the possibility of a hung parliament'. But the Tories are at rock bottom", he said.

So does this mean Mr Sunak could even wait until January 2025 to go to the polls?

"There is a risk the economy will get even worse by November," Sir John said. "I think October is as long as they will have before having to admit the game is up."