Chancellor Philip Hammond said he was disappointed that only Rory Stewart had signed up to a Tory leadership campaign pledge on the public finances.
Mr Hammond warned that the leadership race could lead to candidates promising to spend “money they don’t have” on eye-catching initiatives.
The Chancellor has called on the leadership rivals to commit to keeping the national debt falling every year, and to maintain the current limit of the deficit at 2% of GDP at least through 2021-22.
At an event in the City of London he told the Press Association: “It is a disappointment to me that only one of the candidates remaining in the race, Rory Stewart, has actually signed up to that pledge.
“I would urge the other candidates to do so, to reassure the public that our hard-won reputation for fiscal responsibility is not going to be squandered during the course of this competition.”
The favourite in the race, Boris Johnson, has promised to extend full-fibre broadband to every home in the country within five years, nine years ahead of the Government’s 2033 target.
The UK’s largest broadband infrastructure provider Openreach said providing full fibre technology to the whole of the UK “isn’t quick or easy” and would require a £30 billion investment.
Mr Hammond said: “The ambition to roll out full fibre broadband more quickly is a noble ambition, and I share it.
“But we have worked very hard with the industry partners to get the fastest possible rollout agreed.
“I am not sure that trying to go faster than is technically sensible is going to deliver appropriate returns to the economy.”