The Liberal Democrats are demanding to know whether distribution of the newspapers on polling day last July amounted to an election expense in support of Labour, which should have been declared.
Liberal Democrat Chris Davies beat off a strong surge from third place by Labour's Phil Woolas to take the seat from the Conservatives.
The Representation of the People Act bans spending to "promote the election of a particular candidate" unless it is authorised by the candidate or his or her agent.
A spokesman for the Daily Mirror said: "We don't feel it is appropriate to comment."Reuse content