Multiple industry sources said Government officials have notified the four competing agencies about whether they have been successful in the statutory review.
Omnicom Media Group’s Manning Gottlieb OMD has been vying against Dentsu Aegis Network’s Carat, the incumbent since 2014, as well as teams from WPP's Group M and Publicis Media.
A spokesman for the Cabinet Office, which has been running the review with the Crown Commercial Service, said: "We are currently in the legal standstill period. We intend to issue a confirmation of the agency that will be awarded our new media-buying contract on 22 May."
It is thought that the Government is carrying out some final checks before making a formal appointment.
If Manning Gottlieb OMD is confirmed as the winner, it would likely be the largest UK media account move since the Government appointed Carat four years ago.
The Government was the biggest advertiser in the UK during Gordon Brown’s administration and it is still a top 10 spender behind Sky, Procter & Gamble and BT.
The Cabinet Office has been at pains to run a fair process because the 2014 media review ended up in the High Court after WPP challenged the decision.
When Alex Aiken, executive director of Government Communications, launched the current media review in October 2017, he wrote in Campaign that "transparency" and "openness" would be "at the heart of our renewal process".
He promised the Government would play "a leading role" in tackling problems such as brand safety and viewability and said he wanted to build "a media partnership for the future" with the winning media-buying agency.
The Cabinet Office used PwC to advise on the review and adopted the principles of the ISBA media agency framework contract to improve transparency.
The Government has said quality of service is important and that media pricing would only count towards 10% of its decision, which was widely seen as an acknowledgement that the 2014 review was focused too much on price at the expense of service.
None of the four agency groups involved in the review offered any comment about the review.
Truth, a start-up which styles itself as a blockchain media agency, took part in an early stage of the Government tendering process. However, it is understood Truth has not been involved in the concluding part of the review.
Carat’s contract expires around November 2018.
Tesco, Lidl, Volkswagen Group and Peugeot Citroen owner PSA Group are among the big brands to change media agency in the last four years but they all spend less than the UK Government, which first consolidated its media-buying with a single group in 2010.