The planned demonstration could bring the 'green and gold' campaign led by Manchester United Supporters' Trust into conflict with manager Sir Alex Ferguson for the first time, after the United boss insisted he would accept any protest as long as it did not have a detrimental effect on his team.
So far, supporters have respected his wishes, but the plan for up to 10,000 fans to remain outside the ground at kick-off and burn effigies of the club's American owner Malcolm Glazer - while their team try to build on a 3-2 first-leg lead in a crucial European game - is in danger of crossing the line.
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MUST chief executive Duncan Drasdo insisted last night that it was not an official protest being planned by his organisation, but said he would not try to stand in the way of fans if they wanted to go ahead with it.
'We're not co-ordinating anything but we're not against it either,' said Drasdo. 'People can make up their own minds over how to express themselves but we're not pushing it at the moment. We have other priorities.
'We're more interested in participation in the internet campaign. The green and gold is a better illustration of the campaign for change.'
Last night the number of fans signed up had reached 124,000 as calls grew for the Glazers to bow to public pressure and sell to the Red Knights.
Many of them will be involved in tonight's demonstration which is believed to have been planned back in January.
One MUST member said: 'This is a long time in the making. The idea got put forward a few weeks ago but, in reality, we have been preparing something like this for months.
Green and gold: The protest against the Glazers is set to turn up a notch during Wednesday's visit of AC Milan with supporters demonstrating their displeasure
'Millions of people - maybe even billions - will watch this match and, when they see thousands of empty seats, it will send a message out to the Glazer family.
'I've heard there could be 10,000 involved in this and, although some will return to their seats for most of the match, there could be a large percentage of people who will stay away in protest.'
Wayne Rooney, who scored twice in the first leg at the San Siro, isexpected to be fit for a game that sees his England team-mate DavidBeckham return to Old Trafford.
Beckham, who spent 10 years at United, admitted that it will be the first time he will have wanted his old team to lose.
Hesaid: 'I'm not nervous about the game. But it was always my dream toplay for Manchester United and I never thought I would play againstthem with a different shirt. Now I will be returning as a Milan playerand it will be very emotional for me.
'I will always be asupporter of Manchester United and this will be the first time in mylife that I want them to lose. We all know that it will be a hard gamebut nothing is impossible.'
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Beckham's AC Milan team-mate Pato is also expected to play after the Brazil striker recovered from a thigh injury.
Meanwhile, the father of Standard Liege full back Marnick Vermijl has confirmed the 18-year-old is joining United.
The Barclays Premier League champions have decided to hand Vermijl a three-year contract and pay his Belgian club £200,000 after he impressed on trial.
'They said he was the best right or left back they had seen in years,' claimed his father Dirk. 'Ferguson told me that Marnick in the following years would become a fixture in the Manchester United squad.'
United scouts spotted Vermijl despite the fact he has never played a first-team game for Liege.
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