Barrister Mark McDonald, representing 35-year-old Letby, described the decision to widen the police investigation as a "huge mistake".
Mr McDonald said: "We now have substantial and significant expert evidence which completely demolishes the prosecution case against Lucy Letby and points the finger in a very different direction to that which the police are currently looking."
Last month, at a press conference arranged by Mr McDonald, an international panel of neonatologists and paediatric specialists claimed Letby was not responsible for attacking babies.
Panel chairman Dr Shoo Lee said "in all cases death or injury were due to natural causes or just bad medical care".
The panel's findings have been passed to the Criminal Cases Review Commission (CCRC), which investigates potential miscarriages of justice.
Letby's legal team hopes her case will be referred back to the Court of Appeal which has the power to quash her conviction.
Letby, originally from Hereford, is serving 15 whole-life sentences after being convicted of murdering seven babies, and trying to kill seven others including one on two occasions, at the Countess of Chester Hospital's neonatal unit between June 2015 and June 2016.
Thirlwall Inquiry chair Lady Justice Thirlwall is expected to publish her findings in the autumn.