Sir Keir said the average cost per year is £101m but the net overall cost is £3.4bn, not £10bn, and all public sector projects are measured in net costs.
However, there is confusion over the government's calculations as the full agreement between the UK and Mauritius reveals the UK will pay:
• £165m a year for the first three years;
• £120m for years four to 13;
• £120m plus inflation for every year after to year 99;
• £40m as a one-off to a fund for Chagossians;
• £45m a year for 25 years for Mauritian development.
If inflation were to remain zero for the next century, this would work out to around £10bn over 99 years.
Assuming an average of 2% inflation, Sky News analysis suggests costs could rise as high as £30bn.
Downing Street stood by its figures, saying government accounting principles were applied to adjust for long-term costs and the value of the pound today is worth more than the pound in the future.