Helsinki's City Board had been expected to approved a plan for a new traffic tunnel costing close to 180 million euros and pass the project along to the City Council on Monday.

Instead, Social Democratic Party representatives pushed through a postponement and called for an in-depth study of the impact of the project on traffic flows in the city.

Deputy Mayor Nasima Razmyar (SDP), said the construction of the tunnel should be viewed as an important part of the wider traffic planning of the city, and was surprised by a recent debate in which it has been seen as a separate project from the rest of the city's traffic planning.

The tunnel plan has led to a heated political dispute with the Greens and the Left Alliance strongly opposing the project which has the backing of other parties in city government.

Razmyar justified the SDP's move for a postponement on the grounds that the tunnel has been criticised, among other reasons, for greatly increasing car traffic. She noted that, according to official city reports, vehicle traffic would become impossible in the area if the tunnel were not built.

"This must now definitely be clarified, " she said.

Razmyar added that numerous other projects are planned for the area that depend on the construction of the tunnel.

Construction of the controversial tunnel has been scheduled to begin as early as next year. The 1.6-kilometre tunnel planned in Sörnäinen would is intended to facilitate north-south traffic, especially in the expanding Kalasatama district.

There are currently about 8,000 residents in the Kalasatama district. With the completion of construction in the area, around the end of the 2030s, it is expected to have about 30,000 inhabitants and more than 10,000 jobs.

The current cost estimate for the tunnel is approximately 180 million euros. If the project is approved and moves ahead as planned it could be opened for traffic in the early 2030s.