Manchester United have been left with red faces – again – after another security blunder saw two ticketless supporters sleep undetected inside Old Trafford in a desperate bid to watch the Arsenal match on Saturday.

Sportsmail understands the enterprising pair, thought to be foreign students, took an official tour of the stadium on Friday.

They then hid in toilets at the famous venue, instead of exiting with the rest of the group, and were only spotted the following morning after staging a sleepover at the Theatre of Dreams.

Old Trafford security has been called into question after two fans were able to sleep overnight

The embarrassing episode will raise concerns at the club, who launched a wide-ranging review of security after their Premier League match against Bournemouth in May was abandoned when a bomb, later found to be a dummy device left behind on a training exercise, was discovered in a toilet.

Tour guides count customers in and out while United employ an external company, Controlled Event Solutions, to oversee security at Old Trafford.

Those who go on tours are searched beforehand with metal detectors, along with their belongings - and United say there was no risk to safety.

However, the fact that two members of the public could remain undetected in the stadium for so long hours before a sold-out Premier League match may trigger alarm bells.

The fans were aiming to stay inside the ground to watch United's game with Arsenal

The Red Devils played out a 1-1 draw with the Gunners after a late Olivier Giroud header

United have launched an internal investigation into what happened.

A club spokesman said: ‘We realise people will go to any lengths to watch a game at Old Trafford but these people went too far. The fact that they and their belongings had been searched before entry means we are confident there was no risk to safety.’

A source said: ‘You honestly could not make it up. The two have gone on the tour on the Friday and for some reason, they must not have counted the numbers in and out and they have stayed there overnight without anyone seeing them.’

The source added: ‘After the Bournemouth fiasco they retrained all the staff and beefed up security and searches but then this happens.’

United drew the match 1-1 with the Gunners in front of a near-capacity 75,264,

Police were called after the discovery of the pair, who faced temperature lows of 2 Celsius, but they decided not to make any arrests.

United offer 80-minute tours of the famous venue on a daily basis priced at £18 for adult non-members and £12 for junior non-members.

This isn't the first time security at Old Trafford has been questioned, following a 'bomb' scare earlier in the year

Fans had to be evacuated prior to United's end of season home clash with Bournemouth

A fake 'bomb' device had been left behind after a terror exercise, causing embarrassment for the club and its security team

Sniffer dogs were brought in at the time, before the device was discovered to be a fake

Fans are taken into the players dressing room and get to emerge from the tunnel before picking up a certificate. The club website states: ‘Don’t forget to collect your certificate from the customer service desk in the Megastore...’

The stadium was evacuated 20 minutes before kick-off of United’s final home game of the season against Bournemouth when a fan discovered what appeared to be a bomb strapped to the back of a toilet cubicle door.