Difficult times for the London Ambulance Service as it faces a fresh investigation into its performance.

LAS’s foundation trust application was submitted to the Department of Health on 1 December but it now faces formal investigation by NHS London over its handling of the Day of Action strike on 30 November.

NHS London chief executive Dame Ruth Carnall said the strategic health authority was “very disappointed” with LAS’s performance during the industrial action, “despite early planning”.

Dame Ruth told the SHA’s board last week a report was due in “the coming weeks” and that the trust was working with lead commissioner NHS North West London on how to mitigate risks if there was another strike.

The day saw the Metropolitan Police, St John’s Ambulance and private ambulance contractors drafted in to help as the LAS was left with only half of its fleet staffed.

The trust reported “significant pressure” in east London, the area with the most calls. By 9.30pm that evening the director of operations said there were 100 patients waiting for ambulances, as call volumes between 13 and 18 per cent higher than normal were processed.

Neighbouring ambulance services offered support and GPs were asked to visit some patients to whom the LAS was unable to send a response.

LAS said it would screen calls and try only to respond to “genuine emergencies”, which saw some hospital trusts ask why they could not do that all the time.

The news comes a week after the publication of a report into a software switchover at LAS in June which caused large delays in responding to calls.