A tax on commercial properties is set to increase in April, and critics say it’s time for a bigger change that better aligns with the growth in online sales.

(Bloomberg)—U.K. Chancellor of the Exchequer Philip Hammond will be unable to offer significant concessions for retailers in his budget next month, according to an official familiar with his plans, though Prime Minister Theresa May promised relief for those hit hardest by increased taxes on their stores.

There is little room to help high street traders on March 8 as the scale of reform needed is too broad, the official said. With Brexit set to be triggered by the end of March, Hammond has limited capacity for giveaways, according to the official, who asked not to be identified because the talks are private. May told lawmakers in Parliament on Wednesday she will “make sure there’s appropriate relief” for those taking the biggest hit.

“We want to support businesses,” May said. “I recognize there has been particular concern.”

Business rates, a tax on commercial properties, earn the government about 26 billion pounds ($31 billion) a year, according to real estate advisers Colliers International. A revaluation of the levy, determined by the rental value of the premises, rather than the sales generated by the site, is set to come into effect in England, Scotland and Wales in April.

“The business rates system is out of line with any…