Doubts have been raised as to how widely available Lifetime Isas will initially be when the products start launching from next month.
Several major savings providers said they have no immediate plans to launch Lifetime Isas, with some still going over the details or waiting for finalised rules.
But the Treasury said it expected the market to continue to grow, with more providers coming on board as products were developed.
Lifetime Isas will be made available to the under-40s to open from April 6, to help them save up to £4,000 per year for their first home or their retirement, with a 25% Government bonus.
Over their lifetime, savers could potentially get a maximum bonus of £32,000. Savings can be used to buy a first home, or used for other purposes after the saver reaches 60. But generally, savers withdrawing their money early for reasons other than buying their first home will face withdrawal charges.
Experts have raised concerns that some savers may ditch their workplace pension, which gives them the benefit of employer contributions, in favour of saving into a Lifetime Isa.
Former pensions minister Baroness Altmann told the Press Association: "It is very difficult to see people being better off saving for later life with a Lifetime Isa, rather than a pension."
Hargreaves Lansdown and Nutmeg have both confirmed plans to offer investment versions of Lifetime Isas from April.
But Nationwide Building Society does not currently plan to offer Lifetime Isas from next month.
It said: "We have previously supported government schemes aimed at helping people into a home of their own, such as Help to Buy Equity Loan and Help to Buy Isa.
"As a major savings provider, our members have many and diverse needs and we need to be assured that such products are simple for them to understand."
An HSBC spokesman said: "We are continuing to work through the details of the product."
Barclays said it was waiting for finalised rules from regulators.
Lloyds Banking Group, which includes Halifax, would review its participation when "regulations are finalised".
An RBS spokesman said: "We are supportive of initiatives that help people develop great savings habits, however we are not yet in a position to confirm if/when we will offer Lifetime Isa as part of our savings range."
Yorkshire Building Society said it currently had no plans to launch a Lifetime Isa.
Bonuses will be paid into Lifetime Isas at the end of each tax year, so providers coming on board later this year would still be able to offer the bonus for the 2017/18 tax year.
Jane Ellison, Financial Secretary to the Treasury, said Lifetime Isas would help self-employed people to save for the long-term, which was especially important when they were unable to benefit from a workplace pension with employer contributions.
She said: "We want to make sure savers can start to benefit from the new Lifetime Isa as soon as possible by pushing ahead with the necessary legislation and working closely with industry and the regulator.
"This means that banks, building societies and other financial providers can start signing up.
"We know that the number of providers of the Lifetime Isa may be small to start with, but we expect a number of stocks and shares providers to be signed up from the outset so people can begin to open accounts from April 6 this year, and the first bonuses will be paid by April 2018.
"We anticipate that the market will continue to grow, with more providers, including cash providers, joining over the course of the year as they put their systems in place and develop their products."