Millwall dominated the second half but were unable to make their superiority count as Stephen Hughes' strike plunged the Lions deeper into the mire.
Hughes lobbed home Gareth Williams' pass just after the half-hour to give Leicester a deserved half-time lead.
Millwall pressed through a succession of David Livermore corners, one of which forced Paul Henderson to tip away Paul Robinson's close-range header.
But Millwall couldn't add to their 11 goals at home and remain third-bottom.
Millwall manager David Tuttle:
"If I was a betting man I'd bet on us to stay up.
"We performed well and put them under a lot of pressure in the second half
but couldn't put the ball in the net.
"I will be trying to get a striker in before the loan deadline passes this
week."
Leicester interim manager Rob Kelly:
"We're not safe yet and we must keep our focus.
"This league is unusual with the results recently - look at Sheffield Wednesday beating Preston.
"The players have been magnificent for me and they were again."
Millwall: Marshall, Lawrence, Vincent (Hayles 58), Whitbread,
Paul Robinson, Elliott, Dunne, Livermore, Cameron, May,
Asaba (Williams 31).
Subs Not Used: Doyle, Phillips, Braniff.
Booked: Whitbread, May.
Leicester: Henderson, Kisnorbo, McCarthy, Johansson, Stearman,
Maybury, Hughes, Williams, Gudjonsson, Hume (Welsh 84),
Fryatt (O'Grady 66).
Subs Not Used: Douglas, Brevett, Gerrbrand.
Booked: O'Grady.
Goals: Hughes 31.
Att: 10,523.
Ref: M Thorpe (Suffolk).