The challenge, which was launched by Greater Manchester's night time economy advisor Sacha Lord earlier this month, argues that there is a lack of evidence to justify the delayed reopening of the sector.
In a letter to the Prime Minister, Osmond and Lord called the decision 'plainly irrational'.
The Government's current roadmap out of lockdown provides an indicative date for non-essential retail to reopen on 12 April.
Outdoor hospitality settings will be able to reopen on the same day, however, under the plans indoor hospitality will not be able to unlock until 17 May 'at the earliest'.
In an update at the weekend, Lord said that the Government has until Wednesday this week (17 March) to respond to the legal challenge.
He added that hospitality will need time to prepare for reopening, and as such his team will be asking for the case to be expediated to the High Court.
Writing in the Daily Mail at the weekend, Osmond wrote: "Indoor ‘non-essential’ retail outlets will be allowed to reopen on 12 April, while pubs, bars and restaurants will have to remain shut for another five weeks unless they have a garden or terrace they can use for outdoor dining.
"Not only does this long delay needlessly squander the benefits of the fast vaccination rollout, it is plainly irrational, unscientific and not based on evidence.
"So, along with Sacha Lord, night-time economy adviser for Greater Manchester, I wrote to the Prime Minister this weekend notifying him of our intention to take his Government to court.
"When taking momentous and unprecedented actions affecting millions of its citizens, the Government must base its decisions on evidence, not prejudice, and should be held to account if it does not."
