Deputy Heidi Soulsby says the effect of GST on the index-linked States pension fund hasn't been factored into P&R's calculations.
Deputy Heidi Soulsby has analysed P&R's figures and says the first year of introducing GST - if approved - will cost in excess of £90M.
That's £50M more than P&R's estimate, because of the effect of the tax on the States pension fund, which is index linked:
"If there is any change in the underlying rate of inflation and inflationary effect, which we know GST will have, that will impact on the pension scheme. It'll basically take a chunk out of that fund and, whether it's under or over invested, it's effectively impacting the taxpayer."
"It'll be a one-off deterioration in the funding level of around £50M. That's a big chunk. It'll take the whole cost of GST for the first year to £90M."
Deputy Soulsby is allied with deputies Gavin St PIer and Sasha Kazentseva-Miller over their approach to public finances. They call themselves the Fairer Alternative and intend to put forward their own plans ahead of next week's debate:
"I suppose you could call it scenario four, after their three. It puts together a balanced package which we think is pragmatic and realistic, particularly at this time in the States term."
Deputies will debate GST for a second time next week, beginning a day earlier than normal on Tuesday 17 October.
P&R's first attempt to bring it in was defeated in February.
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