M&S Bank Arena Liverpool said it was conducting an investigation into the incident - later revealing it was caused by a sensor being "triggered".

The concert had already been rescheduled from its original date of 13 October, when it was postponed due to issues with the venue's power supply.

An S Club fan, who was at the concert, told Sky News the band were performing when "it all went black" and a message played over the tannoy telling people in the arena to "evacuate immediately" and there was a "report of fire in the building".

"Most people went out but some people stayed in their seats and had to be told to leave," said the fan, who asked not to be named.

She said concertgoers were able to return to the arena after about five minutes but they were not given any further information about what caused the evacuation.

She added that she was concerned during the incident due to heightened fears since the Manchester Arena bombing.

There had been reports by concertgoers that there was no option to leave via a marked fire exit.

Concerns were also raised that security checks were not carried out as fans re-entered the arena.

Faye Dyer, chief executive of the ACC Liverpool Group, which runs the arena, apologised to concertgoers, saying "we understand the customer experience was not in line with our high standards on this occasion and for that we are sorry".

"We wish to thank the band and the audience for their patience and co-operation," she added.

"We are proud that incidents of this nature remain rare over the past 15 years of hosting successful live events."

Ms Dyer said that while the building's safety systems "responded appropriately", the arena lights "did not automatically turn on as programmed which meant our manual systems were activated, resulting in a short delay".

She added: "We can confirm that at no point were fire exits blocked. However due to the location of the activation, our stewards directed individuals away from some arena floor fire exits towards others, while the source of the activation was investigated.

"Customer security checks were carried out prior to the show and - as customers remained in very close proximity to the arena for a short amount of time, on private land monitored by CCTV and with additional visual checks put in place for re-entry - the decision was made for them to return to the arena so the show could resume.

"A full check of the arena and systems was carried out in the meantime with the event deemed safe to continue."

In its initial statement on Monday night, the arena said the evacuation was triggered by a "minor incident" - but did not disclose after further details.

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S Club, formerly known as S Club 7, kicked off their 25th anniversary reunion tour with a show in Manchester on 12 October.

But less than 24 hours later, they were forced to cancel the original Liverpool date, saying on Instagram that they were "beyond devastated".

They told fans the venue had "an unforeseen technical issue impacting their power supply and therefore your safety".

S Club, which now features Rachel Stevens, Jon Lee, Tina Barrett, Jo O'Meara and Bradley McIntosh, head to the US for concerts in Chicago, Los Angeles and Boston next month.

Band member Paul Cattermole died in April aged 46 from heart problems - shortly after the concert dates were announced.

His bandmate and ex-girlfriend Hannah Spearritt quit the tour several weeks later.