The Parliamentary Budget Officer says the federal government has not budgeted enough money to buy between six and eight Arctic patrol ships.

A new report by the PBO says the government's $3.1-billion budget is only enough to pay for four ships.

But the PBO adds that could drop to three ships if there is a delay of more than a year.

Last week, industry and military sources told The Canadian Press the government has scaled back its original plan to buy between six and eight vessels, choosing instead to buy five with an option for a sixth.

The deadline to sign a deal to begin constructing the vessels is the end of the year.

The Irving shipyard in Halifax was selected in 2011 to build the ships.