The British and Irish Lions' tour of South Africa could be postponed if restrictions in place because of the coronavirus pandemic mean international fans cannot attend, says South African Rugby chief executive Jurie Roux.
In July, organisers said the tour would go ahead as scheduled, with the first Test on 24 July 2021.
But Roux said it is not "commercially viable" if supporters cannot travel.
"There has been some talk of moving it out [to a new date]," Roux said.
"But our travel advice is that by June/July we should be at what is deemed to be normal international travel. But we are monitoring it on a monthly basis."
He added: "Travelling for international events will likely still be under more pressure than pre-Covid [in July 2021].
"No spectators and people not being able to travel would not make this commercially viable and then we would discuss how we continue with the tour."
A Lions spokesperson said: "As recently announced, we are fully intending for the Tour to go ahead as scheduled, but, as you would expect, SA Rugby, in conjunction with the Lions, are contingency planning for all eventualities."
Registration for the ticket ballot begins on 2 September, with Roux saying "the bulk of the tickets" will be for South Africa fans.
He added that full refunds for tickets and package tours would be offered if the tour is cancelled.
Full 2021 tour schedule:
Saturday 3 July: Lions v Stormers - Cape Town Stadium, Cape Town
Wednesday 7 July: Lions v South Africa 'Invitational' - Nelson Mandela Bay Stadium, Port Elizabeth
Saturday 10 July: Lions v Sharks - Jonsson Kings Park, Durban
Wednesday 14 July: Lions v South Africa 'A' Team - Mbombela Stadium, Nelspruit
Saturday 17 July: Lions v Bulls - Loftus Versfeld, Pretoria
Saturday 24 July (first Test): Springboks v Lions - FNB Stadium, Johannesburg
Saturday 31 July (second Test): Springboks v Lions - Cape Town Stadium, Cape Town
Saturday 7 August (third Test): Springboks v Lions - Emirates Airline Park, Johannesburg