Ambulance crews will now have to contact their central control room before taking any patients to an under-pressure hospital's A&E department.
Northwick Park Hospital is the only one in London with such controls in place.
It has also written to local GPs asking them to refer patients to other hospitals to help it cope.
London North West Healthcare Trust said it was working with London Ambulance Service and NHS England to find out why so many patients were going to its A&E.
The hospital has been struggling to cope since two nearby A&E units, at Central Middlesex and Hammersmith, closed in September.
Its board meeting on Wednesday heard that in September ambulances had to wait on 179 occasions for more than 30 minutes to hand over a patient, while 30 patients had to wait more than an hour. The national target is 15 minutes.'Absolutely disgusting'
Board papers blame a "poor level of staffing on wards" and say the "primary cause of breaches of national target is a lack of beds."
Muhammed Butt, Labour leader of Brent Council, said: "It's absolutely disgusting that my constituents are going to be suffering - they have to travel so much further.
"It's putting their lives at risk and we warned about this; that beds should be in place, the A&E should have the proper staff and the hospital should have the proper number of beds. They're not in place and God knows when they would be in place."
In December, the hospital will gain an extra 20 beds, another 24 in February and a further 60 the following year.
In the House of Commons, Prime Minister David Cameron said more A&E consultants and nurses were being recruited for north west London.
He added: "Both Hammersmith and Central Middlesex hospitals have GP-led urgent care centres on site that are open 24 hours a day, seven days a week."
Earlier this month a report found that patients requiring emergency treatment at the hospital had the longest waits in the country.
In August the Care Quality Commission said some patients were being discharged too early because of patient flow through its A&E department.
Watch the BBC video here: