Informa share count update to alter FTSE indices from start of trading on October 12, 2026

FTSE Russell has confirmed index adjustments linked to an updated share count for Informa plc, with the changes scheduled to take effect from the start of trading on Monday October 12, 2026. The announcement follows Informa’s recent primary placing and an RNS update that altered the company’s shares in issue, prompting recalculation of index weightings across the FTSE UK Index Series.

What is changing and why it matters

The adjustment affects several headline UK indices. According to index notices filed on October 7, 2026, the FTSE 100, the FTSE 350, the FTSE All-Share and the FTSE 350 Lower Yield Index will all reflect Informa’s revised share count from the opening of trading on October 12. The procedural update is a technical consequence of a primary placing that increased the number of shares outstanding and therefore alters Informa’s market capitalisation and weight within free float weighted indices.

For investors, fund managers and index providers, the practical effect is straightforward. Passive funds and exchange traded funds that track those FTSE indices will rebalance to incorporate the new weightings. That can produce predictable flows into or out of the underlying stock and repositioning across the index, depending on whether Informa’s weight rises or falls after the new shares are included.

Likely market impact and timing

Because the change is scheduled to coincide with the start of trading on October 12, the most intense trading flows are likely to be concentrated in the opening session. Index-tracking funds routinely adjust holdings in line with index provider notices to maintain tracking accuracy, and market makers and arbitrage desks will price in the expected action before the market opens. The result can be a short window of elevated volume and modest price movement as passive and active managers rebalance.

The magnitude of any price effect depends on the scale of the placing and the adjustment to Informa’s free float market capitalisation, relative to the liquidity of the stock. In larger cap names that sit in the FTSE 100, even small percentage moves in index weight can require tens of millions of pounds of trading by funds that apply strict index-following rules.

Who is affected

The direct beneficiaries and losers are holders and trackers of the affected indices. Passive UK equity funds and ETFs that replicate the FTSE 100, FTSE 350 or FTSE All-Share will need to update holdings. Active managers that use index weightings as a benchmark may also adjust exposure to limit tracking error.

Other market participants such as liquidity providers, prime brokers and trading desks will be active in pre-market pricing to ensure that large rebalancing orders can be absorbed with minimal disruption. For some index-linked derivative contracts and funds, the reweighting could alter hedging and collateral requirements for the remainder of the quarter.

Context and what to watch next

This update is part of routine index maintenance that follows corporate actions. Index providers publish these technical notices when share counts change as a result of placings, rights issues, buybacks or other corporate events. The London Stock Exchange Group’s index division, FTSE Russell, issues effective dates so market participants can prepare for the operational and trading consequences.

Investors should monitor trading volumes and Informa’s intraday price action on October 12 for signs of how the market is absorbing the reweighting. Fund managers and investors who want to avoid the short term volatility that can accompany index rebalances may choose to stagger trades or use algorithmic execution to minimise market impact.

Beyond immediate trading effects, changes in share count and index weight are a reminder that corporate financing decisions can have outsized effects on institutional flows. Primary placings increase capital available to a company but also dilute existing shareholders. When those companies are components of widely tracked indices, the ripple effects extend to thousands of funds that benchmark to UK indices.

Market participants will also be watching for any follow up announcements from Informa or FTSE Russell that clarify the scale of the adjustment or outline related corporate actions. For now, the mechanical index change provisioned for the start of trading on October 12, 2026 remains the key development for UK equity index trackers and the investors they serve.