The tactical manoeuvring in the Rugby World Cup ends today as the competition effectively reaches its knockout stage a week early with all four home unions facing crunch matches that could see them on the first plane home unless they can pull results out of the fire.

England should have the easiest task tonight, with a win over Tonga enough to let them them stay on and take their defence of the crown they won four years ago to at least the quarter-finals, though a draw would be enough to make it an historic weekend for their opponents. Wales are in a better position in their match against Fiji, with a draw enough to put them through, while Ireland need a near miracle to come up with the four-try win that would see them qualify and eliminate Argentina.

Of all the matches, the one that is hardest to predict is the one in St Etienne when Scotland take on Italy. Ninth and tenth in the world rankings, they have each won two and lost two of their match-ups since the last World Cup and have both beaten Portugal and Romania in this competition while losing to New Zealand.

Both sides unveiled their teams yesterday, with Italy being forced into the biggest change of plan when Marco Bortolami, their captain and lock, was ruled out by injury. While the loss of their iconic leader is bound to have an effect on the team, they have a ready-made replacement in Alessandro Troncon, the 100-cap scrum half.

Frank Hadden, the Scotland coach, who rested all his star players at the weekend against the All Blacks, responded by sticking with the side that demolished Romania in the second round of pool games, meaning that he is keeping faith with the experimental centre combination of Rob Dewey and Simon Webster, despite having the greater experience of Andy Henderson and Marcus Di Rollo available.

The match will be won and lost up front and there Hadden has made no bones about going for his biggest unit, with the 20st Jim Hamilton partnering Nathan Hines in the second row and Ross Ford, the 17½st hooker winning the nod over both his squad rivals.

“This is something that we have been looking forward to for a long time,” Hadden said, adding that the selection process had been so long and complicated that it had even forced him to delay a training session while the final bench positions were hammered out. “We have prepared extremely well, though we have every respect for the Italian team and we know it will be a competitive and uncompromising game. We know they have lost their captain but one reason they have made so much progress in recent years is that, like us, they have improved their strength in depth.”

So for all four countries, it is “game on” this weekend. The choice is stark, either win – and in Ireland’s case win well – or be on the plane home and watching the rest of the World Cup on television. It is that simple, knock out or be knocked out.