Jose Mourinho's comments appear to be aimed at Manchester City.
Manchester - City have dismissed those reports as an "organised and clear" attempt to damage their reputation
Jose Mourinho says it is vital that UEFA clamps down on Financial Fair Play infringements to level the playing field and enable Manchester United to become serious Premier League title contenders again.
Mourinho's comments appear to be aimed at Manchester City, who are the subject of an ongoing investigation by European football's governing body into alleged FFP breaches.
In November, German magazine Der Spiegel said the Premier League champions had broken rules that dictate how much money owners can put into a club, having purportedly obtained documents from the whistleblowing outlet Football Leaks.
City have dismissed those reports as an "organised and clear" attempt to damage their reputation, while UEFA president Aleksander Ceferin said on Tuesday UEFA's investigation would be concluded shortly.
UEFA could ban City from next season's Champions League in the event of discovering breaches it believes to be serious enough.
Mourinho did not mention City directly but made clear he believes that United are not currently competing on even terms at the top end of the Premier League.
United have not won the league since Sir Alex Ferguson retired in 2013 and Mourinho acknowledged that he did not know how long it would be before they became champions again.
"I don't know. It depends on our evolution but also the evolution of others," he said, speaking ahead of Saturday's home match against bottom club Fulham.
"If the ones above us keep going in same direction and if their ambition and investment is continuous, that's one thing. Another thing is if they stop, or if the Financial Fair Play makes them stop and then we can close the gap a little bit better."
Calculations made from publicly available figures suggest that since the summer of 2016, City have spent around £527 million ($671 million) on players, while United's outlay has been about £379 million.
Asked for his thoughts on the likelihood of City being penalised by UEFA for over-spending, Mourinho said: "The world is full of suggestions and I never know if they are true or not true, and that's not my job to analyse that."
The United manager has acknowledged that improving on their current league position of eighth is going to be difficult because he is short of natural goalscorers.
Romelu Lukaku has scored just once for United since September 15, and was left out of the starting line-up for Wednesday's 2-2 Premier League draw at home to Arsenal, as Mourinho went with a front three of Anthony Martial, Marcus Rashford and Jesse Lingard.
Mourinho was impressed with the work ethic of all three players but suggested that none can be relied on as a regular source of goals.
"They work very hard but it's not about work," he said. "It's about natural qualities.
"We need to score more goals. I think one thing is team dynamics, with routines adapted to the qualities of the players.
"But in the end the most important thing is putting the ball in the net, and goals, points and victories."
Mourinho was reluctant to discuss publicly the reasons for Lukaku's loss of form but said he is in regular dialogue with the striker as to how to get him back to his best.
"That is something I feel comfortable to discuss with him, which I do," he said. "I am not so comfortable to discuss with you.
"I think it is quite an easy job for you to analyse a player and performances, you have knowledge, experience, eyes and data. If you mix all this information, I think you can analyse it."