The health watchdog said it had taken "urgent action" after an unannounced inspection at Pro Medicus in Harpenden.
It has declined to say why it took the action, stating its reasons will be revealed in a report being published at a later date.
Pro Medicus director Stephen Burton said it had taken "robust action".
The CQC said it carried out an unannounced inspection on 3 and 4 August and chose to take "urgent action to temporarily suspend the registration of this provider".
It said that the suspension was due to last until 18 September "whilst they take action to address the concerns we have".
The CQC said the provider could not undertake any of its regulated work while it was under suspension.
Pro Medicus has more than 30 years' experience providing independent ambulances, medical supervision and first aid training courses throughout the UK.
Its website claims its clients include NHS trusts, Stevenage Football Club, Cambridge United Football Club and Saracens Rugby Club.
The watchdog gave the company a clean bill of health in its last inspection in November 2013.
Mr Burton declined to say what the issues were about, adding: "We've taken advice from the CQC. A number of things were highlighted - we're not denying what they've said need addressing.
"We've taken robust action and we've got staff in at the moment going through everything and we're updating lots of things.
"As far as we're concerned we've always had a good name. We're going to bring it up to scratch and hopefully in the next few weeks everything will be in place."

The producers of 20% of the world's oil and gas say they share the ambition to limit warming to 2C.
They promise to work to reduce the greenhouse gas intensity of the global energy mix.
But green groups were dismissive, saying that "arsonists don't make good firefighters".
The Oil and Gas Climate Initiative represents major producers including BP, Shell, Saudi Aramco and Total among others.
In their declaration they call for an "effective climate change agreement" at Cop21 - the 21st UN conference on climate change, which takes place in Paris at the end of November, when 196 countries will attempt to reach a new deal.
"Our shared ambition is for a 2C future," the 10 chief executive officers said in a statement which acknowledges that the existing trend of the world's net greenhouse gas emissions is not consistent with this aim.
"It is a challenge for the whole of society. We are committed to playing our part. Over the coming years we will collectively strengthen our actions and investments to contribute to reducing the greenhouse gas intensity of the global energy mix."
The companies say they will attempt to make their own production operations more efficient and say that over the past 10 years they have already reduced their emissions by 20%.
They promise to promote natural gas as a better option than coal and invest in carbon capture and storage as well as renewable energy.
"Sometimes in all these discussions you have the impression that all fossil fuels are the bad guys. But the bad guys are part of the solution," Total's CEO Patrick Pouyanne told a gas and electricity summit in Paris earlier this week.
"Whatever people think, we still need fossil fuels. We need to make advocacy for gas. We need to explain to our policy makers that gas has to be encouraged," he told news agencies.
However the group of 10 does not include major US oil companies such as Exxon and Chevron.
Environmental campaigners were quick to pour scorn on the oil and gas producers' initiative, saying it would do little to aid the decarbonisation of the global economy.
"The oil companies behind this announcement have spent years lobbying to undermine effective climate action, each and every one of them has a business plan that would lead to dangerous global temperature rises, yet suddenly they expect us all to see them as the solution, not the problem," said Charlie Kronick from Greenpeace.
"The world should thank them for their offer of advice but politely turn it down. Arsonists don't make good firefighters."
Climate negotiators reassemble in Bonn next week in an effort to advance a new global deal. They will have their first opportunity to respond to a slimmed down draft that is expected to be the basis of the Paris agreement.
Follow Matt on Twitter @mattmcgrathbbc.
A private ambulance firm based in Hertfordshire has had its registration temporarily suspended due to "concerns" by the Care Quality Commission (CQC).
[NEXT_CONCEPT]
The leaders of 10 of the world's biggest oil companies have offered their qualified support for a new global treaty on climate change.