Mayer Brown said the move was a direct result of lobbying from the pensions industry.
Trustees had been given until March 31 to comply with the regulations, however, this has now been extended to May.
The firm said the postponement was designed to enable potentially affected parties to consider the guidance and their subsequent need to register as a trust or company-service provider or not.
Partner Jonathan Moody said: "Most paid trustees know they are not in business, but still, criminal penalties are a concern and they feel pressured into registering just to be on the safe side.
"Often it is not the cost that worries them but the ongoing obligations they might then have to comply with as some kind of watchdog. We have voiced our concerns for some time over this. But we should be grateful HMRC has finally listened, even though it is at the 11th hour."
A statement on the HMRC website said the MLR 9 registration guidance for trust or company service providers will be updated in April to reflect feedback from businesses in their discussions with HM Treasury about the interpretation of the regulations as they apply to TCSPs.
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