FTSE Russell publishes the results of its quarterly review of the FTSE UK Index Series after the London close on 2 September, with changes taking effect on 21 September. Entain, the owner of Ladbrokes and Coral, is widely expected to be relegated from the FTSE 100 to the FTSE 250. Persimmon is named alongside it as the other likely casualty. Entain closed at 513.2p on Friday 28 August against a 52-week high of 915.6p.
That is, on the face of it, an unusually clean thing to price. There is a discrete outcome, a fixed announcement time, a named candidate, a second named candidate, and a resolution source that is public, unambiguous and published by the index provider itself. Most contracts that trade heavily have less structure than that.
No venue lists a market on it.
Deposit $20, get $50 to trade
Welcome bonus on the world's largest prediction market.
18+ · New users only · Availability and terms vary by region · Trading involves risk of loss · BeGambleAware.org
What is and is not there
Searching Polymarket's public index returns nothing for Entain, nothing for Persimmon and nothing for reshuffle. Kalshi's catalogue of 13,633 market series contains no series mentioning Entain, Persimmon, the FTSE, the London Stock Exchange, UK equities or gilts.
That could mean the category does not exist, and it is worth saying plainly that it is not the explanation, because the interesting part of this is what both venues do list.
Kalshi runs quarterly index-removal contracts right now. The series KXNDXREMOVEQ, "Nasdaq-100 quarterly removals", carries eight active markets in the Financials category, settling off Nasdaq's own published constituent list. They are named, per-company questions. One of them is "Will Workday be removed from the Nasdaq-100 in Q3 2026?" Alongside it, KXSP500ADDQ carries twenty-one active markets of the form "Will Venture Global be added to the S&P 500 in Q3 2026?", settling off S&P Global.
So the contract does not need inventing. "Will Entain be removed from the FTSE 100 in Q3 2026?" is the same sentence as the Workday one with two nouns changed, resolving off an equivalent public page maintained by an equivalent index provider, on a review that happens on the same quarterly cycle.
Polymarket runs one too, and it is a removal. Its event on whether MicroStrategy is delisted from an MSCI index carries three markets, one per deadline, and the December contract is open as this is published. It resolves on an official MSCI announcement. Polymarket also ran an event asking which companies would be added to the S&P 500 in the first quarter of 2026, seven markets deep, one per candidate company, and has an open contract on whether SpaceX joins the index this year.
The asymmetry runs both ways
The gap is not that either venue ignores the FTSE, and it is not that either venue ignores index composition. Each of them does one and not the other, and they do not do the same one.
Polymarket has priced the FTSE's level, and the FTSE's membership not at all. A search returns fifty events for the index, carrying sixty-three markets between them: FTSE 100 (UKX) Up or Down on February 23?, What will FTSE 100 (UKX) hit in March?, and more of the same. Every one of them is about where the index goes. Not one is about which companies are in it.
There is a second thing worth noting about that set, which is that all fifty are closed. Their end dates run from October 2025 to April 2026, and Polymarket currently has no open FTSE market of any kind. We checked that against subjects we knew were live on the same instrument: the S&P 500 returns open contracts and the Premier League returns twenty-six of them, so the search does distinguish open from closed. Polymarket priced this index daily for about six months and has since stopped.
Kalshi prices index membership and covers the UK, separately. It has forty-eight series with a UK subject, including a snap election announced before 2027, who leaves Starmer's cabinet, and which wealth taxes the UK announces. It has the Nasdaq-100 removal series described above. It has nothing where those two sets meet.
Both venues hold both halves of this contract, and on both of them the halves are live at the same time. Polymarket has an open contract on a named company being removed from a named index, and no open contract on the FTSE at all. Kalshi has eight open contracts on named companies being removed from a named index, and forty-eight open UK series that touch politics and tax and never the stock market. The sentence will this company be removed from this index by this date is being traded on both venues this week. Neither of them has written the London version of it.
Why this is not the same as a contract with a gap in its wording
Earlier this week we looked at a Polymarket contract whose resolution text was silent on an edge case that the ballot could plausibly produce, and found that the venue had answered the question by listing a second contract rather than amending the first. That is a market that exists and does not cover something.
This is the other shape. It is not a contract silent on an edge case. It is an edge case with no contract at all, and the thing that makes it worth writing down is that the absence is not explained by the product being unavailable, the resolution source being unclear, or the subject being outside what the venues cover. All three of those are demonstrably fine.
We do not know why it has not been listed, and we are not going to guess. Listing decisions are commercial ones taken by operators for reasons they do not publish, and inferring a rationale from an absence is exactly the move that produces confident wrong answers. The observation is the asymmetry, not the motive.
A note on what this event is not
There is a tempting story here, and it is worth naming so that nobody writes it: that a gambling company dropping out of the FTSE 100 says something about betting migrating to prediction markets.
It does not, and nothing here is evidence for it. Index membership is a ranking by market capitalisation against the rest of the index, and a company's place in that ranking moves for reasons that have nothing to do with where anybody places a bet. Why Entain's has moved is a question about Entain, and this piece is not about Entain. It is about the fact that a clean, dated, publicly resolvable event has no contract on it.
The editorial take
The useful question this raises is not about Entain. It is about what determines whether an event gets priced.
The intuitive answer is that priceability does the work: a discrete outcome, a hard date, a clean resolution source. This event has all three in unusually good order, and there is no contract. Meanwhile the same venues run contracts on how many times Elon Musk posts in a week, which has none of the tidiness and considerably more resolution risk.
So priceability is not sufficient, and probably is not the binding constraint. What the binding constraint actually is, we are not in a position to say. But it is worth knowing that the gap exists, because the next time somebody argues that prediction markets will price anything with a verifiable outcome, this is a counterexample sitting in plain sight with a live comparator on the same exchange.
Frequently asked questions
Is there any market anywhere on Entain leaving the FTSE 100?
Not that we can find. Polymarket's public search returns no results for Entain, Persimmon or the reshuffle, and Kalshi's catalogue of 13,633 series contains none mentioning Entain, Persimmon, the FTSE, the London Stock Exchange or UK equities. We verified the searches against subjects we knew existed first, because a search returning nothing for everything is not evidence of anything.
Do index-membership contracts exist at all?
Yes, and they are live on both venues right now. Kalshi runs a series called Nasdaq-100 quarterly removals with eight active named-company markets, one of which asks whether Workday will be removed from the Nasdaq-100 this quarter, settling off Nasdaq's published constituent list. It also runs twenty-one active markets on companies being added to the S&P 500. Polymarket has an open contract on whether MicroStrategy is delisted from an MSCI index, resolving on an official MSCI announcement, and another on whether SpaceX is added to the S&P 500 this year.
Why has neither venue listed the FTSE version?
We do not know, and we are not going to speculate. Listing is a commercial decision taken by an operator for reasons it does not publish. What can be said is that the usual explanations do not apply here: the product format exists on both venues, the resolution source is a public page maintained by the index provider, and both venues already cover the UK and the FTSE in other forms.
