When it reopens a limited service will run on weekdays
London Mayor Boris Johnson has been accused by opposition parties of trying to make political capital out of the reopening of the East London Line.
The first phase of the £1bn extension, agreed in 2003 by the former Labour administration, should open next week.
Labour called the timing "a stunt during an election campaign" while the Lib Dems said the mayor's office was "seeking political party advantage".
The mayor's office said it was conducting "business as usual".
The parties reacted after the reopening, which was scheduled for Thursday, was put back to next week.
Transport for London (TfL) said the delay was for "operational reasons" as safety checks needed to be carried out.
The extended East London Line will become a part of the London Overground network.
A spokesperson for Mr Johnson said: "Under the pre-election guidance the GLA and TfL are expected to conduct business as usual.
"This is the long-awaited opening of a railway that will benefit 100,000 Londoners daily and certain publicity is required to make the trial of the new services effective.
"The guidance provides that it is appropriate to mark this opening with a non-political event."
Labour's leader on the London Assembly, Len Duvall, said: "Boris Johnson is in chaos over his stunt to hold an official event using taxpayers' resources during an election campaign.
"He's been trying to take credit for a project delivered with £1bn of investment under a Labour government and a Labour mayor."
The Lib Dems have written a letter of complaint to Peter Hendy, the Commissioner of Transport for London.
Caroline Pidgeon, Lib Dem member of the London Assembly, said: "You do not need politicians to publicise this success story for TfL.
"If you wish the event to go ahead then it should but with no politicians."
Initially a limited service of eight trains an hour will run between New Cross Gate and Dalston Junction on weekdays between 0700 BST and 2000 BST, with full evening and weekend services beginning from 23 May.