Patients' lives could be put at risk after Brexit unless there is progress on continued research co-operation with the European Union, Labour is warning.
Shadow health secretary Jonathan Ashworth will say it is "utterly unacceptable" for there to be uncertainty about the arrangements after Brexit.
He will also promise that Labour would ensure medics and carers from other EU states will continue to be able to come and work in the UK after it has left the bloc.
Ashworth will visit Brussels in the latest sign of Labour's push to build links with EU figures.
He will say Labour would seek to remain part of the European Medicines Agency (EMA), prioritise the negotiation of continued access to reciprocal healthcare schemes or a comparable alternative and stay part of the Horizon 2020 agreement on research funding.
He will vow Labour will not "sign off on a Brexit deal that turns the clock back on medical innovation" or means patients face longer waits for new treatments.
Theresa May used her Mansion House speech to indicate she wanted the UK to remain part of the EMA and called for a "far-reaching science and innovation pact" with the EU.
Setting out his Brexit "red line," Ashworth will say: "Given the scale of trade between the UK and the EU on medicines, but perhaps more fundamentally given disease knows no borders, it would be great folly to dismiss the huge benefits that the UK and the EU 27 have gained from our close relationship over the past 40 years.
"It is utterly unacceptable to put patient safety at risk because of lack of certainty about medicine regulation post-Brexit.
"Slow progress on reaching a deal could mean delays accessing potentially life-saving treatments, harming patient and public health in both the UK and EU.
"Labour will not sign off on a Brexit deal that turns the clock back on medical innovation or sees patients in the UK having to wait longer to get access to life-changing treatments.
"It is a red line for me and it is a red line for the Labour Party."
The Conservatives dismissed Mr Ashworth's claims as "opportunistic scaremongering."
A spokesman said: "While we will seek to remain part of the European Medicines Agency, we have been clear that no matter what people will of course still have access to the medicine they need."