She said weaker economic growth forecast over the next few months was likely to hit tax receipts, adding to pressure on government finances.
"With the PM announcing a partial U-turn on the cut to winter fuel payments, the dilemma faced by the chancellor over how to deal with increased spending pressures in environment of low economic growth and high interest rates hasn't gone away," Ms Gregory said.
Matt Swannell, chief economic adviser to the EY Item Club, said: "Talk of the reinstatement of some winter fuel payments and the likely need to spend more on defence will further increase the pressure for tax rises."
On Wednesday, Prime Minister Sir Keir Starmer announced plans to ease cuts to winter fuel payments, in a U-turn following mounting political pressure in recent weeks.
In her October 2024 Budget, Reeves had introduced £40bn in tax increases, the largest since 1993. However, at the time she pledged there would be no additional tax rises beyond those already announced.
The chancellor is following two main rules, which she has argued will bring stability to the UK economy:
Reacting to the latest borrowing figures, Chief Secretary to the Treasury Darren Jones said: "After years of economic instability crippling the public purse, we have taken the decisions to stabilise our public finances, which has helped deliver four interest rate cuts since August, cutting the cost of borrowing for businesses and working people."