Alison Munro, the chief executive of High Speed Two Ltd said: "This is a project that the country needs. The existing railway is becoming increasingly full and by the middle of the next decade parts of it will be effectively full.
"So we need to start now to plan to have that capacity in place."
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Omid Saidy, 20, was knifed to death after asking a drug dealer and another man to leave the area, police say.