World Rugby officials are confident Japan will deliver a successful 2019 World Cup and say there is no 'Plan B' to move the tournament to another host.

Bernard Lapasset, president of rugby's governing body, spoke to ease Japanese concerns on Tuesday, saying his organisation was "working well" with local authorities to deliver a successful event, despite recent setbacks.

"Japan 2019 was difficult for us with the stadium, but now we are confident working with them," the Frenchman told reporters at Twickenham Stadium, ahead of Friday's start of the 2015 World Cup.

Japan's proposals for the 2019 tournament were placed in jeopardy when Prime Minister Shinzo Abe scrapped plans for a new stadium in July. Costs for the stadium, which was due to be the centerpiece for the 2020 Tokyo Olympics as well as the venue for the Rugby World Cup opening game and final, had spiralled to $2.1 billion, nearly twice original estimates.

The huge costs sparked outrage and prompted Abe to scrap the design, leaving no time for a new venue to be built. Japan 2019 organisers are looking at Tokyo Stadium as a replacement, but with a capacity of 50,000, it could be considered too small for World Rugby. The final of the 2015 edition will be hosted at the 82,000 Twickenham.

Lapasset said that further meetings with organisers were scheduled for later this week, and that alternatives were not on the table.

"Plan B? That is the wrong message," he said. "We have no Plan B for 2019. We are very confident of working with Japan. It is four years to go now, and time to complete the process.

"The stadium was a big change, but now we progress to resolve this. There is no Plan B and [there has been] no contact with any other country."

Last month World Rugby gave Japan until the end of September to provide 'formal reassurances' on the tournament's viability following the loss of the stadium. That led observers to speculate that other nations might be in running to step in as hosts, with South Africa and Australia the favourites.