And the Kiwi Test ace is now a major doubt for the Carnegie Challenge Cup semi-final against St Helens in 10 days' time.

Webb's injury is a massive blow for Rhinos ahead of their most crucial spell of the season so far.

Coach Brian McClennan revealed: "Brent injured his back at training and it is very doubtful he will be going away this weekend.

"We will have to see how he pulls up, but at this stage it is very unlikely he will be travelling over to France with us."

Webb has missed just two games for Rhinos this season – both in the Challenge Cup – and would have been a key weapon in their armoury against Super League's two most in-form sides.

McClennan said it is too early to say whether Webb will recover in time to face Saints.

"I don't know yet," he admitted. "Back injuries are very hard to assess, in terms of how long the recovery will be – especially in the early stages.

"Meirion (Jones, physio) is taking care of him, so he's getting good care. We'll know more about whether he'll be available for the semi next week."

McClennan revealed hooker Matt Diskin (elbow) and prop Ryan Bailey (ankle) are also set to miss the trip to France and are cup doubts.

Both players have undergone scans after being hurt in last Friday's win over Huddersfield Giants, and Rhinos are awaiting the results.

"We are 90 per cent sure Disko won't be playing this week and 100 per cent sure Bails won't be," said McClennan, who will not reveal his travelling party for France until tomorrow, when Rhinos are due to fly out.

But he confirmed teenage full-back Ben Jones-Bishop will come into the squad as cover for Webb.

Front-rower Nick Scruton has competed a three- match ban and McClennan said: "He is back and that is a positive for us."

The injuries to Webb, Diskin and Bailey have come at the wrong time for Leeds, but McClennan insisted: "It is all part of football. We will move on. That's what we've done all year and we'll carry on doing that."

Rhinos reserve grade centre Kallum Watkins and hooker Paul McShane were due to fly to Australia today with the England Academy squad.