Manchester United has a major problem with ticket touts.

Club officials say it has led to intimidation and threats against staff.

Stories have been recounted of fans left in tears after discovering the seats they have bought for games at Old Trafford do not exist, having been sold by touts, sometimes multiple times.

For the Premier League game at Liverpool, it was discovered someone paid £900 for a ticket in the away section.

The resale of tickets is illegal in the UK but the same legislation does not apply to companies abroad.

The club is trying several approaches to combat the problem but their powers are limited and accept there are times when it appears the people they are trying to stop are one step ahead.

Additionally, as they have discovered, some of the actions they do take are met with fierce resistance from their own supporters.

Speaking to United staff earlier this month, the scale of the problem was laid bare.

In the past, a feature of an Old Trafford matchday were the people dotted around Sir Matt Busby way offering to sell tickets. But numbers have dwindled.

Now trading is done on Facebook or on WhatsApp groups and in a way that makes formal identification hard or impossible. Infiltration is incredibly difficult.

In the minutes of October's Fans Forum meeting, United said 2,000 tickets for the Premier League game with Chelsea on 20 September were "successfully reclaimed" and "over 4,500 active users were blocked" from buying tickets. The club believes the use of bots to purchase huge number of tickets under assumed names is the major problem.

On average, the club says, season ticket holders return between 5,000 and 7,500 tickets per game for "repurposing". They estimate between 10% and 15% of United's 47,000 season tickets are owned by touts, who take over tickets from holders who do not want to keep them. Valid addresses are no longer required because the tickets are delivered in digital form.

Around the first five home games of the season, United either blocked or cancelled 22,000 tickets, a figure the club regards as the "tip of the iceberg".

The club places checks on suspicious tickets that require fans to seek out officials at the ticket office at away games. From that, it was established that one fan paid £900 to for a ticket to see the win at Liverpool, another paid £800 for the 2-2 draw at Tottenham earlier this month.

With more tickets available, some of the scenes at Old Trafford can be upsetting as supporters discover they have been duped. For away matches, the ticket holders are allowed into the game. The source of the ticket is logged and action taken. At home, it is frequently not possible when tickets for the same seats are being sold multiple times.

Touting is big business. It is estimated it is worth hundreds of thousands of pounds a year to those involved and is not restricted to Manchester United. The biggest northern clubs and all those in London are the main targets, with overseas supporters, willing to pay a premium to circumvent club ticketing policies they either have no knowledge of or whose criteria they do not meet.

At one point, Manchester United sent non-permanent staff out to buy tickets off the street. Within minutes, it became apparent those running the touting operation had worked out what was happening and demanded to know why. On-street ticket checks have led to verbal and physical threats.

The club feels it is starting to get across who exactly is in charge of its season tickets. Doing the same with a membership of around 500,000 - all members are able to apply for tickets - is impossible.

Tickets for corporate areas, which rarely sell out, have been reduced in price meaning paying inflated sums to touts is unnecessary.