Rangers manager Mark Warburton has told BBC Sport that Celtic need more competition in the Scottish Premiership in order to succeed in Europe.
Celtic have won the league title in Scotland for the past four seasons and are on course to clinching the championship yet again this campaign.
However, the Bhoys have failed to do much of note under manager Ronny Deila in Europe so far.
The Hoops have failed to qualify for the Champions League for the past two seasons, and they may not progress to the knockout stages of the Europa League this campaign.
Rangers boss Warburton believes that for Celtic to succeed in Europe, they need to be tested week in and week out domestically.
"The better the challenge you face, the better the opportunity to develop and move forward,” the Englishman told BBC Sport. "Any team, in any league, you require the quality, consistent challenge."
Rangers are playing in the Scottish Championship at the moment, but they are on course to gaining automatic promotion to the Scottish Premiership at the end of the season.
Warburton believes that the Gers could provide the competition for Celtic that they are currently lacking in the Scottish top flight.
"I'm sure Celtic need that quality of challenge on a more consistent and regular basis to allow them to be successful at the high level,” said the 53-year-old. "I'm talking here as coming in from the outside.
"The European teams have that variety and quality of challenge and we have to make sure that Scottish teams have equally as good an offering of fixture."
Rangers are five points clear at the top of the Scottish Championship table at the moment, while league Celtic are three points ahead of their nearest rivals in the Scottish Premiership with a game in hand.