The Health and Safety Executive is examining a complaint about the competency of contract staff covering for striking firefighters in London.
Private firm AssetCo was brought in by the London Fire Brigade as part of contingency plans to deal with the walkouts by the Fire Brigades Union.
More than 5,500 London firefighters have completed two eight-hour strikes in a dispute over new contracts.
The brigade said it was assisting the Health and Safety Executive.
An HSE spokesman said: "We are looking into a complaint which alleges that AssetCo staff have not been trained to do the work they have been asked to do and might therefore be putting themselves at risk."
Private contractors responded to 20 call-outs, including eight fires, between 1000 and 1800 GMT on Monday, London Fire Brigade (LFB) said.
AssetCo - which says it provides fully outsourced fire and rescue services, including the provision of personnel, training and equipment - signed a £9m seven-year deal with LFB last year.
Bonfire Night strike
An LFB spokesman said: "The FBU raised concerns with the HSE about the training supplied to contingency fire crews and they are therefore obliged to respond.
"The HSE are carrying out a review that we are assisting with. We have provided them with all the documentation they have asked for and no issues have been identified at this time."
The company had used 27 fire engines and 170 contract workers during the eight-hour strike on 1 November.
The FBU said it called the walkout as staff faced the sack unless they agreed to new rotas.
The dispute centres around the brigade asking firefighters to sign the contracts to bring new shift patterns into effect.
The union is scheduled to go on a 47-hour strike planned to start on Friday, which is Bonfire Night and also the Hindu, Jain and Sikh festival of lights, Diwali.
It is traditionally the busiest night of the year for fire services.