In the letter, Khan puts his support behind raising the threshold for the retail, leisure, and hospitality grant to apply to venues with a rateable value of up to £150,000.
This has been the objective of many campaigners in the pub trade including the Raise the Bar campaign, launched by business improvement districts (BIDs) across the country.
He implored the Chancellor of the Exchequer Rishi Sunak and Secretary of State for Business, Energy and Industrial Strategy (BEIS) Alok Sharma to protect businesses that are “falling through the cracks”.
He said almost 30,000 London businesses would have received grants if they were located anywhere else in the country.
Data from the Altus Group estimated almost a quarter of pubs, across the country, would be left out of the grant scheme because their rateable value was too high.
Operators said this made them fear the future of their business and were apprehensive to turn to loans to keep them afloat.
Khan also asked the Government to introduce a rent compensation scheme for small and medium-sized businesses struggling to pay rent.
He said the Coronavirus Job Retention Scheme should be improved to help businesses back into action, with workers able to go back to work on reduced hours with their wages topped up by the Government.
Workers who have seen their income fall below the national living wage should also be supported, Khan said.