The UK Foreign Office's about-face on proscribing the IRGC as a so-called "terrorist" entity come despite its approval by the Home Office, The Times reported on Thursday.
Citing sources in the government, the report added there are also concerns about how to blacklist the IRGC because, unlike other proscribed bodies, the Iranian force is an official government agency.
"Foreign Office officials have real concerns about proscription because they want to maintain access. The Home Office, and the government more broadly, supports the move. The IRGC should have been proscribed by now but the whole process is on ice," a Whitehall source said.
The report comes some 10 days after the European Union also opted not to blacklist IRGC, despite calls from the European Parliament.
European Union foreign policy chief Josep Borrell said on January 23 that the block could not list the IRGC as a "terrorist" entity without an EU court decision.
Speaking before a meeting of EU foreign ministers in Brussels, Borrell said a court ruling with a "concrete legal condemnation" had to first be handed down before the bloc itself could apply any such designation.
"It is something that cannot be decided without a court, a court decision first. You cannot say I consider you a terrorist because I don't like you," Borrell told reporters, stressing that the court of an EU member state had to issue a concrete legal condemnation before the bloc could act.
A week earlier, the European Parliament adopted an amendment, calling on the EU and its member states to include the IRGC on their terror list. It also passed another resolution later, calling for more sanctions against Iranian individuals and entities and putting the IRGC on the EU terrorist list over alleged human rights violations during the recent riots.
The European Union, however, imposed sanctions against a number of Iranian individuals and entities for what it claimed to be a crackdown on recent riots, which were triggered after the death of a young Iranian woman of Kurdish descent in Tehran in September last year.
Iranian intelligence agencies say foreign spy services, including the CIA and MI6, have been behind the riots which claimed dozens of lives from people and security services.
Tehran, meanwhile, strongly slammed the efforts for blacklisting the force in European countries, noting that the IRGC is an official government body of the Islamic Republic of Iran and hence such a move is in contravention of international laws and principles.
Iranian officials also stress that if it was not for the efforts of the IRG forces in the fight against terrorism in Syria and Iraq, Europe would not have its current security.