Manchester United's hopes of Carlos Tevez undergoing a medical tomorrow are in limbo after they failed to secure permission for the tests from West Ham.

The Argentina striker flew in to Manchester tonight and United had been preparing to put the 23-year-old to go through a series of fitness and medical checks.

However, one club is not allowed to give a player a medical unless his current club agree and although United have made West Ham aware of their wishes, the London side will not give formal permission for a medical until they have been made - and accepted - an offer.

West Ham remain adamant they hold Tevez's registration, that he is contracted to them for another three years and that they have unilaterally terminated all third-party agreements with Tevez's representative Kia Joorabchian.

Joorabchian maintains his company Media Sports Investments (MSI) own the economic rights to the player, who has already agreed personal terms with Manchester United.

United chief executive David Gill, who is in the Far East on the club's tour, insisted yesterday a medical would take place this week but said the deal had become "over-complicated".

Gill said: "Effectively, we are trying to work with four parties - the player, West Ham, the company who own Carlos' economic rights and the Premier League - in order to sort it out.

"The process is ongoing. The player will have a medical at some point this week, then we will take it from there. We still have to sort out the paperwork but we are confident that can be achieved in the course of the next week or so."

United are understood to have agreed an initial loan deal for Tevez with a view to a permanent transfer, a similar deal to that which took Javier Mascherano from West Ham to Liverpool in January.

That hit a stumbling block however because in January, West Ham cancelled Mascherano's registration.

The Premier League are not allowing the club to do the same procedure with Tevez on the basis that such a move would indicate the original agreements with Joorabchian were still binding despite assurances to the contrary.

The Argentina striker last week submitted a written request to West Ham asking to be released from his contract but the Hammers stuck to the Premier League's advice and refused to do so.

The League have made it clear they expect a significant chunk of any transfer fee to remain with West Ham rather than be passed on to Joorabchian as compensation for breaching the contracts he had with the club.