The extension of M1 line continued in 2005 with the start of the construction of of tunnels and three stations linking St Nedelya Square and the Interped World Trade Center in Izgrev (station Frédéric Joliot-Curie). 2006 saw the start of the construction of another section of the same line (consisting of of tunnels and three stations) linking Izgrev and Mladost I housing estate. The completion of the first three stations was projected for the autumn of 2007, however as a result of various delays it was the second section from the first line (Vasil Levski stadium – Mladost 1) that first entered into service on 8 May 2009, operating for a brief period of time separately from the north-west portion of the line. The remaining section between Serdika and Vasil Levski stadium station finally entered into service on 7 September 2009 establishing an uninterrupted link between Obelya and Mladost 1 stations.