The Football Association suspects Manchester United have withheld key information about their England internationals because José Mourinho and his staff do not fully trust the governing body to prevent it being leaked to rival clubs.

Gareth Southgate, the England manager, is trying to improve the relationship between the FA and the relevant people at Old Trafford after admitting there had been a trust issue, citing “the perception of [information] being leaked”.

Southgate was referring to the sports science data the FA requests for international get-togethers so he can examine detailed analysis about the players he has called up. Every top-division club agreed to supply this data for Euro 2016 but that has not continued at Old Trafford since Mourinho took over last summer.

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Other clubs are also said to have misgivings, although United are the only one to be identified, and Southgate described it as “a case of building up trust” and, specifically, making it clear to the relevant people that the information would be treated as confidential.

“To clarify, we share our data,” Southgate said. “We don’t always receive data, though. That’s what we would like because it would help us work out a training schedule when they [the players] first come into camp. That can be a big help but we can’t enforce it. That has to be an agreement – sharing – and at the moment some clubs are more comfortable with it than others. One of the dangers, I guess, is the perception of it being leaked.”

Building that trust has not been easy because the FA has employed a number of staff in key positions who also have jobs with Premier League clubs. Sir Alex Ferguson was never comfortable about Arsenal’s physiotherapist, Gary Lewin, being part of the England set-up for many years and, more recently, Southgate has added Steve Holland, one of Chelsea’s first-team coaches, to his staff. Lewin, who spent six years with England until his departure last year, was named as West Ham’s new head of medical services on Tuesday.

Other roles have been given to Manchester City personnel and the suspicion of the FA is that United, and others, are concerned about the possibility of information reaching rival teams.

Southgate has set about the process of removing what the FA calls itinerant staff. Holland, for example, has left Chelsea to take on a full-time role as Southgate’s assistant. Martyn Margetson, England’s goalkeeping coach, still has his role at Crystal Palace but various others, including City’s head of sports science, Sam Erith, and the masseur, Mark Sertori, will not be involved for the games against Scotland and France.

Six months after taking the job, Southgate said he was generally encouraged by the help he had received from managers – “relations are as healthy as you could expect” – and when it came to Mourinho he knew United were happy with the decision to keep Marcus Rashford out of the European Under-21 Championship this summer. Southgate had previously said he wanted Rashford to have the experience of playing in another tournament and the England manager admitted he had opened himself to allegations of caving in under pressure from United. “I knew when I made the decision it would be one of the things that would be fired across – ‘you have done it because of José’ and so on – and in many respects it would have been easier for me just to send him [Rashford] with the under-21s and not have to face all of that. But I had to make a decision that was the right one.”

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That decision, he explained, was based on Rashford’s high appearance rate for United, with Southgate using the example of Harry Kane’s diminished performances in Euro 2016 after playing for the under-21s the previous summer and barely getting a rest. “The key for us is not the relationship with José, it’s the relationship with Marcus. I have not spoken to him [Mourinho] about it but I am not in a position where I can make decisions to keep people happy. If I am trying to keep José happy that’s going to be, let’s say, fairly difficult.”

The same might apply to Wayne Rooney, and Southgate, having rung England’s record scorer in March to explain why he had been left out of the previous squad, said he had not felt it necessary this time. “I’m mindful of being respectful, so I don’t like talking about him in the past tense but equally I have to make decisions based on form and what’s right for the team. We have to allow other people to come through. We have exciting players with potential who now need to take responsibility.”