The ECB had previously stated that fees for trades passed over the harmonised platform would be set at 15 cents per transaction based on "conservative assumptions" about volume throughput.

But in a quarterly update on the project published by the ECB, Jean-Michel Goddefroy, chairman of the T2S board, says that settlement volumes reported by the 23 CSDs that have committed to the system are around 18% lower than was anticipated in 2010 for this period.

"This gives the T2S Board cause for concern, as one of the underlying assumptions of the T2S pricing policy was that EU volumes would be no more than 10% lower than the projected volumes," he says. "The Board will keep a close eye on this matter and explore all possible ways to avoid increasing fees for T2S users."

In another blow to the project, the ECB has once again extended the deadline for migration to November 2016 in a bid to alleviate concerns about operational safety issues. The initial plan called for the central securities depositories (CSDs) of the Clearstream and Euroclear groups - which account for around two thirds of T2S volumes - to migrate as part of the same wave.

However, this scenario met with strong opposition from several users who were concerned about the severe operational risks and potential for market disruption from a botched migration.

Under the revised scheme, the 23 CSDs to have committed to the project will relocate their data to the T2S infrastructure in three waves within 18 months from the June 2015 go-live date.

The first wave include the Bank of Greece securities settlement System, Italy's Monte Titoli, Malta Stock Exchange, Romania's Depozitarul Central and Switzerland's SIX-SIS.

Belgium's securities settlement system NBB-SSS, Euroclear ESES, Spain's Iberclear, Portugal's Interbolsa and Slovakia's CDCP will join from July 2016.

From November 2016, Clearstream Banking (DB1Gn.DE), Austrian OeKB, Euroclear FIN, LuxCSD, Danish VP Securities and its subsidiary VPLux, Estonia's AS Eesti Väärtpaberikeskus, CSD of Lithuania, KDD Slovenia and Keler Hungary will join.

T2S programme manager Marc Bayle says: "Migrating to T2S is a complex issue, given the number of actors, the volume of operations, and the risks at stake. For this reason, it was decided from the early stages of the project that T2S migration would not happen as a big bang...in terms of wave composition, the operational risks are now more balanced."