Boris Johnson has claimed it would cost more than £150m and take at least nine months to hold referendums on Brexit and Scottish independence in 2020.
The prime minister said Labour will have to adhere to Nicola Sturgeon's "demands" for another "divisive referendum" on Scotland, while Labour want to give voters a final say with another referendum on the UK's EU membership.
Mr Johnson, who will launch the Conservatives' Scottish manifesto on Tuesday, insisted his party will deliver on the Brexit vote and deliver on the "people's priorities".
The Conservatives' own analysis claims it would take a minimum of nine months to hold both referendums, costing £155m. Their estimates suggest a Brexit referendum re-run would cost around £138m, while Scottish independence vote would cost £17m.
The EU referendum in 2016 cost taxpayers £129.1m, the Electoral Commission revealed last year, with the main cost - £94.5 - covering the cost of running 40,000 polling stations, issuing postal votes and counting votes. Meanwhile the Scottish independence referendum in 2014 cost just over £15.8m, taking the combined cost to £144.9m.
The Conservatives warned that giving the vote to 16-year-olds within nine months could cost up to £500 million or result in an additional delay of at least six months to pass the necessary legislation and allow the Electoral Commission to register them.
The PM said: “A majority Conservative government would get Brexit done and focus on the people’s priorities – such as increasing funding in our NHS and reducing the cost of living.
“The alternative is Jeremy Corbyn, a man who can’t even make up his mind on Brexit, submitting to a pact with Nicola Sturgeon, and we already know what terms she will demand – another divisive referendum on Scottish independence alongside a second vote on Brexit.
“The financial cost of this to taxpayers up and down the country will be in excess of £150 million.
“But the real cost will be much, much higher: the chaos of two referendums in 2020 grinding the country to a halt and the world’s greatest political union reduced to the status of a bargaining chip.”
Combined cost of last EU and Scottish independence referendum
Mr Corbyn has revealed Labour plans to give voters a final say on Brexit in a second referendum - with a "credible" leave option and Remain on the ballot - and promised to resolve the issue within six months.
He also said he plans to remain "neutral" ahead of any second vote.
He added that he would not support a second Scottish referendum vote within his first two years in office if he takes over as prime minister.
Ms Sturgeon drew up her red lines, saying a vote on Scottish independence would have to happen next year if she were to support a Labour government.