Once heralded as the partnership to break Mercedes' dominance of F1, the McLaren-Honda alliance is set to be officially terminated this weekend to mark the end of one of the most ill-fated marriages in the sport's history.

Sifting through the debris of the alliance's disastrous union, here are the five critical reasons why McLaren have had to serve notice on their engine partners...

Reason 1: Performance

The fatal problem in the McLaren-Honda reunion, more than 20 years after they ruled the sport, was the unequal split in performance the two protagonists brought to the partnership.

Whereas McLaren believe they've built a car equal in chassis performance to Mercedes and Ferrari's, the Honda engine remains the weakest on the grid - and by a considerable margin.

Worse, it is also the least reliable. As Fernando Alonso summarised during winter testing, "We have only one problem which is the power unit. There is no reliability and there is no power."

Six months later, that damning assessment still rings true. Italy accounted for McLaren's fourth double DNF of the year.

Had Honda failed to deliver in year one of the partnership it would have been forgiveable. But not in year three and certainly not when the Honda power unit has gone backwards from year two.

Failed promises, fading potential, and, to use the description of team boss Eric Boullier after the Canadian GP, "unacceptable" performance.

McLaren's patience could only last for so long.

Reason 2: Personnel

Alonso, McLaren's number one asset, is out of contract at the end of 2017 and the team admitted mid-summer they were facing "mission impossible" to persuade the Spaniard to stay after three podium-less seasons.

"If we are not competitive then he will go away," Boullier conceded to Sky Sports in Spain.

It seems as though the carrot of a Renault engine has persuaded Alonso to stick it out for at least another season. Even if the Renault engine is the second-weakest on the grid, Red Bull have achieved one win and a series of podiums with it this year. And McLaren believe their chassis can compete with with their former title rivals'.

And then there is the wider workforce at Woking to consider after their efforts to build a car to rival that of Mercedes and Ferrari - a mission they believe they have successfully fulfilled - have been rendered irrelevant by the fatal lack of engine grunt.

Without the drastic action of immediate divorce to offer their workforce overdue reassurance their efforts wouldn't go to waste again in 2018, McLaren may not have been able to stem a crippling braindrain from Woking.

Reason 3: Prestige

Ron Dennis once compared McLaren to Manchester United. So try to imagine Manchester United out of Europe and scrambling around at the bottom of the Premier League and the ignominy of McLaren, 20-time world champions, ninth from ten in the Constructors' Championship becomes painfully apparent.

McLaren are too big, too powerful and too proud to have continued like this.

"What you also have to consider is the McLaren brand - this isn't just an F1 team - this is a big group that relies on the success of the racing team to provide wealthy people in the market for a supercar the mental link between owning a McLaren and winning. It's vital to the business that owning a McLaren remains a desirable thing rather than buying into the successful Ferrari or Mercedes AMG brands instead," says Sky F1's Ted Kravitz.

The team will accused of short-termism in switching to a customer supply of Renault power having bet big on an exclusive partnership with Honda delivering long-term gains. But their biggest failing was surely the scale of their ambition in gambling that a Honda works deal was the only way to win championships.

That ambition has now been overtaken by the urgent necessity of rescuing prestige, pride and podiums.

Reason 4: Prizemoney

It's been frequently asserted through the marriage that McLaren couldn't afford to divorce Honda due to the amount of money the Japanese firm have pumped into Woking. The figures vary, but £50m a year is recognised as a conservative estimate. But Honda's failure to deliver has also come with a price attached - and one which doesn't make the benefits of their largesse quite so compelling.

"When you actually look at the impact of loss of FOM money and loss of sponsorship, it starts to diminish the commercial benefits of what Honda bring," executive director Zak Brown said. "And when you net it out, it doesn't have quite the commercial benefit it might appear from the outside."

Reason 5: Philosophy

McLaren's marriage to Honda was underpinned by the team's absolute conviction that - to quote Ron Dennis - "no team is going to win a world championship in the future unless it is the dominant recipient of an engine manufacturer's efforts. I can't understand why everybody doesn't appreciate the simple fact that you aren't going to win a world championship if you have a second-string engine - it's just not going to happen."

The pain, in other words, would be worth the aggravation.

Three miserable years later, however, and the team think differently.

"Do I think you can win with a customer engine? I think you can," said Brown in June.

To some extent, and probably a very large one, the U-turn on philosophy has become a matter of practical necessity; McLaren have been losing too much power, prestige and prizemoney not to change course.

But what might also have changed McLaren's thinking is the lack of changes around F1's engines over the last three years. Whereas works status offered huge advantages for the critical task of incorporating a brand new type of engine at the start of F1's hybrid era, the accumulated knowledge of the last three years has significantly lessened the disadvantages of a customer supply.

A McLaren-Renault partnership isn't likely to be a title winner in 2018. But the team are convinced they have the car-making capabilities to challenge Red Bull, another Renault customer, for at least third place in the Constructors' Championship.

Ultimately, with their faith and patience with Honda extinguished, their decision boiled down to the probability of another year at the back of the grid against the motivating potential of fighting for podiums and, on a good day, wins in 2018.

Put like that, it was an easy decision for McLaren in the end.