UPDATE 1-UK investigators enter London office of firm at centre of Facebook data storm

data storm@ (Updates with officials arriving at office)

LONDON, March 23 (Reuters) - Investigators from Britain's data watchdog on Friday entered the London offices of a data analytics firm at the centre of a storm over allegations it improperly harvested Facebook data to target U.S. voters, a Reuters witness said.

About 20 officials, wearing black jackets with "ICO Enforcement" on them, arrived at the central London offices of Cambridge Analytica, soon after a High Court judge granted a search warrant sought by the Information Commissioner's Office (ICO).

The officials, who were let into the building by security guards, brought crates with them, the Reuters witness said.

Elizabeth Denham, head of the ICO, sought the warrant after a whistleblower revealed Cambridge Analytica had harvested the private information of millions of Facebook users to support Donald Trump's 2016 U.S. presidential campaign.

Britain is investigating whether Facebook, the world's largest social media network, did enough to protect data.

"We are pleased with the decision of the judge, and we plan to execute the warrant shortly," an ICO spokesman said soon after the judge granted the warrant.

Efforts by the ICO to investigate Cambridge Analytica had hit a snag on Thursday after a judge adjourned its application to search the British consultancy group's office by 24 hours.

U.S. and European lawmakers have demanded an explanation of how the British consulting firm gained access to the data in 2014 and why Facebook failed to inform its users, raising broader industry questions about consumer privacy.

Facebook Chief Executive Mark Zuckerberg said on Wednesday that his company made mistakes in mishandling data belonging to 50 million of its users and promised tougher steps to restrict developers access to data. (Additional reporting by Philip George in Bengaluru Editing by Adrian Croft and Catherine Evans)