A north west London hospital set to accept additional emergency patients is already operating at "maximum utilisation", a report has revealed.
St Mary's Hospital will take Hammersmith Hospital's patients when its accident and emergency department closes in September.
The NHS report said additional space could not be found before the change and the plans carried a financial risk.
MP for Hammersmith, Andy Slaughter, said it was "grossly irresponsible".
Health Secretary Jeremy Hunt announced in October plans to close Hammersmith Hospital and Central Middlesex's A&E departments as part of the Shaping a Healthier Future programme.
As part of it Hammersmith Hospital will lose its A&E capacity and become a specialist hospital, operating a 24-hour Urgent Care Centre (UCC) which generally cares for walk-in patients.
Patients requiring an ambulance will instead be taken to St Mary's in Paddington.
'Dangerous and irresponsible'
The Imperial College Healthcare NHS Trust report said: "The current capacity at St Mary's for acute admissions is already at maximum utilisation.
"[St Mary's] will receive an additional 25 ambulances and up to 15 UCC-referred patients a day converting to 13 admissions, and therefore plans for additional capacity are being developed."
But it added that plans to find additional space was "unlikely to conclude prior to the closure of the EU (emergency unit)".
Mr Slaughter said: "Closing Hammersmith A&E… with four months notice, and by their own admission that they are not ready with alternative services, is dangerous and irresponsible and we are going to do everything we can to stop that happening."
He added he had urged Hammersmith and Fulham Council to speak to the NHS Trust to "persuade them to go back and look at this again".
'Better for patients'
The report also said there was a "clinical risk in recruitment and a financial risk" as junior medical staff would have their contracts terminated on 6 August meaning the hospital would have to rely on locums for a month.
Dr Mark Spencer, deputy medical director for the NHS in London, said: "What we're doing in each case is moving the small teams of doctors that are managing the A&Es at Hammersmith or Central Middlesex and moving them to larger units to provide better care over 24 hours a day which is much better for patients."
Under the government's plans, nine hospitals in north-west London will be redesigned so the most serious emergencies can be dealt with at A&Es in Hillingdon, Northwick Park, West Middlesex, Chelsea and Westminster or St Mary's hospitals.
The Trust must now approve proposals to close Hammersmith Hospital and Charing Cross Hospital A&E departments on 10 September.