In a close election, it goes without saying that every result counts. In a general election as close as this one, we might be waiting up to 18 hours after the polls close to discover which party has got the most seats.

This is because it’s unlikely we’ll hear declarations from all 650 constituencies before the lunchtime of the day after the election.

If 2010 is anything to go by, results could still be trickling in on Friday afternoon. Hackney North & Stoke Newington had been expected to declare around 4am on Friday morning; instead the result came in at 3.30pm, the same time as Dudley North, which had been expected to declare as early as 2am.

There are a few reasons why this year’s results could be similarly held up.

The main one is simply the amount of time it will take to sort and count ballot papers. Local and mayoral elections are taking place across almost the whole of England, though not in London. Even in areas where councils use separate ballot boxes for local and general elections, verification of both sets of ballots is required before any counting can begin.

Then there has to be a verification of a sample of the postal votes. This is a requirement that was introduced in the Electoral Administration Act of 2006, which specifies that at least 20% of all postal votes must be checked against identification numbers on the electoral roll, and that this has to take place only when polls have closed to accommodate any late submissions of postal forms.

In 2010 the combination of these various processes, plus the number of places holding local elections, meant that by 4am there had been only 313 declarations from parliamentary constituencies: fewer than half the House of Commons.

Another reason for hold-ups could be because councils have decided to start counting on Friday morning instead of Thursday night. This is not as widespread as it used to be, however. The Constitutional Reform and Governance Act of 2010 specified that all returning officers must hold counts within four hours of polls closing other than in exceptional circumstances. This meant that at the 2010 election only 22 councils began counting on Friday.

It’s tempting to moan about councils not bothering to start until Friday morning, but it’s worth remembering overnight counting in general elections began only as recently as 1950.

Declarations can also be held up by disorganisation or sheer incompetence. Nick Clegg witnessed this first hand in 2010, when his result in Sheffield Hallam was held up until breakfast time on Friday thanks to repeated delays. At one point it was feared proceedings would drag on so long they would disrupt early morning swimming lessons in the leisure centre where the votes were being tallied.

Breakfast time was actually one of the most eventful periods for results at the 2010 election. It was when Caroline Lucas was announced as the winner in Brighton Pavilion, becoming the UK’s first Green MP; when BNP leader Nick Griffin was defeated heavily by Labour’s Margaret Hodge in Barking; and when the Tories failed to take two of their top Labour London targets, Westminster North and Hammersmith.

Nearer to polling day we’ll get more information about which councils are counting on Friday and which are likely to take the longest to declare. Northern Ireland used to meet both these criteria, but in 2010 it counted on Thursday night for the first time and will probably do again this time. It was DUP leader Peter Robinson’s defeat in Belfast East that provided one of the first real shocks of the 2010 election night, coming at 12.50am.

This year I’d imagine the size of some constituencies might mean several seats in south-west England won’t report until Friday, and possibly some of Scotland as well: two key areas on the electoral map, being respectively the location of some of the top Lib Dem-Conservative battlegrounds and of the SNP’s advance on Labour. As such we could be waiting well past the 18-hour mark to learn who exactly holds the balance of power, and by how many seats.

The very last constituencies to declare in 2010 were East Ham and West Ham at 8.30pm on Friday evening. The last seat outside London to declare was Devon West & Torridge at 4.15pm.

In 2010 the Queen had reportedly insisted on a “cooling off period” on the morning after the election, saying she was not intending to see any party leader until after 1pm. I doubt any leader will be in a position to go to the palace by 1pm this year.