Northwick Park and St Mark's hospitals are part of the North West London NHS Trust
Patients requiring emergency treatment in north-west London had the longest waits in the country.
London North West Hospital Trust failed to see 32.2% of its patients within four hours during the week ending 19 October, and 26.7% the following week.
Labour London Assembly member Dr Onkar Sahota said the two "dangerous" results could be connected to the closure of two local A&E departments.
The Trust said it understood "where change is required".
The figures obtained by the Labour Party show the trust performing below the national average, which stood at 11.2% and 9.4% respectively during the two weeks highlighted.
They revealed 827 and 628 patients had to wait more than four hours for emergency treatment.
In a statement the trust said: "We are working with our healthcare partners... to address specific issues relating to capacity, attendances and delays in discharging patients from hospital.
"Agreed actions are in place to improve performance which includes plans for 70 additional beds at Northwick Park Hospital."
Dr Sahota, a practising GP, said the closure of Central Middlesex and Hammersmith A&E departments in September had put west London hospitals under "unbearable pressure".
The trust also looks after Ealing and Northwick Park emergency departments.
The GP said: "Despite consistent warnings, the mayor and government have refused to recognise the dangerous impact these closures have had.
"We can only hope that with the message now devastatingly clear, they will take urgent action to help get A&E services in North West London back on track."
An NHS spokesman said: "The much-needed changes to the health service in North West London will put patients at the centre of their local NHS, with more accessible, 24/7 frontline care at home, at GP surgeries, in hospitals and in the community.
"This means more money will be focused on high-quality patient care."
In August the health watchdog the Care Quality Commission said staff across the trust "were found to be caring and compassionate" but that some patients at Northwick Park Hospital were being discharged too early because of the patient flow through its A&E department.
BBC London News