Mr. Hammond told lawmakers that if Britain could strike a deal with the European Union, there would be a “double Brexit dividend” as certainty returns, and that the government has the leeway to spend money being held back as a contingency in case of a chaotic departure next March without any deal.

But Mr. Hammond’s financial plans are generally based on the assumption that Britain negotiates the type of exit he wants from the European Union: not only a smooth departure, but one that maintains close economic ties to the bloc. On Monday, he said that as a precaution he was “retaining firepower to intervene if the economy needs more support,” and that, if the outlook changes, he might hold another full budget in the spring.

Hard-line Brexit supporters within Mr. Hammond’s Conservative Party want a cleaner break with the bloc, and some are threatening to vote against any Brexit deal that ties Britain too closely to the European Union. So his comments will be seen as a reminder to them of the economic — and political — consequences of anything that risks an orderly departure from the bloc of the type he wants.

They know that Britons are weary of years of cuts to key public services that were introduced as the country battled to stabilize its finances following the financial crisis. Indeed, many analysts believe the vote for Brexit was partly a reaction to austerity policies, particularly in areas of the country that did not benefit from globalization.

Critics argue that Mr. Hammond is still not planning to be aggressive in reversing those policies, and that some cuts will continue, particularly to capital spending.