Under the revised proposals, the Belgian, Dutch and French branches of Euroclear will press ahead six months later than originally planned, onboarding in September 2016.

In the wake of Euroclear's reversal, Clearstream faces an almost year-long delay and will not join the system until February 2017.

In a statement, Clearstream says: "Clearstream will adjust its operational and market readiness activities to adapt to the changes in the migration schedule and remains dedicated to fully leveraging the benefits of T2S to the advantage of our customers."

Announcing the new timetable for the eight-year old project, Mehdi Manaa, T2S project manager at the ECB, says the hiccups in scheduling have led to a complete review of the deliverables calendar, which will see some user testing activities taking place in parallel in order to provide adequate time for all parties to complete the timetable.

"The new composition of the T2S migration waves is considered feasible from a technical perspective," he says. "However, owing to the increased expected volumes and number of migrating actors in the fourth wave, we will be faced with a tighter timeline and fewer buffers to cope with unexpected delays. This requires serious preparations both individually by each T2S participant and collectively by the user communities joining T2S in the same migration wave."

The $1 billion project is viewed as a critical element in the European Commission's ambitions to unite Europe's unruly mishmash of domestic settlement systems and provide a roadmap for an integrated financial market. The ECB is already contemplating a a merger of the Target2 real-time gross payment system with T2S in order to provide a single platform for the processing of cash and securities across the Eurosystem.

In a recent presentation on the Eurosystem's 'Vision for 2020' blueprint, ECB board member Yves Mersch stated: "Over the coming years, we will seek to draw on the synergies between Target2 and T2S. One idea is to consolidate some components of the technical infrastructure and maybe even form a single platform."

Mersch says that such a move would enable Target2 to benefit from some of the state-of-the-art features of T2S, such as the implementation of ISO 20022 standards, and in the provision of a liquidity savings mechanism in Target2 and statistical tools to support banks with their regulatory reporting.