Scotland will fight plans to reduce the Six Nations to a six-week tournament.
England's Rugby Football Union hopes to remove one of the two fallow weekends when games are not played, to create space for the new global season.
But Scottish Rugby's chief executive Mark Dodson says such a move would be a threat to player safety.
"My view is that seven to six weeks is a retrograde step and it's a dangerous step as far as player welfare is concerned," Dodson told Eyes On Events Scotland.
Dodson also insists the unions in Ireland, Wales and Italy share his view.
"We're going to look after our boys," he said.
"Anybody who has just witnessed the intense physicality in this last Six Nations and wants to reduce that down to six weeks is taking a huge gamble with player safety.
"There will be discussion over the next month and we're very clear of how we feel. I'd imagine other people will be equally clear and will share our views."
'Smaller nations will be compromised'
Ian Ritchie, the RFU chief executive, plans to lobby for change when the review of the championship takes place in April, suggesting the new format is introduced after the 2019 World Cup.
Dodson also shot down the proposal on the basis of player resources.
"Nations with smaller player pools are going to be compromised and hit far harder by a six-week competition," he added.
"So you're going to make the Six Nations championship less competitive at a time when the tournament is at an all-time high in terms of popularity and broadcast revenues."
Scotland won three games in this year's tournament for only a second time in the Six Nations era, and the first since 2006, but still finished fourth for the second year running.