They also pointed out that forecasts from the Treasury and US investment banks - which have helped bankroll the Remain campaign - 'have been wrong again'.
Countries borrow money by issuing gilts or government bonds. The cost of government borrowing tends to fall when investors see the country issuing the gilts as a safe bet.
Rates tend to rise when investors are worried and demand a greater return to compensate them for taking on more risk.
The Treasury has predicted the cost of borrowing will rise sharply if Britain decides to quit the EU.
In its report - dubbed the 'dodgy dossier' by Eurosceptics when it was released last month - it warned that interest rates on ten-year gilts could rise by as much as one percentage point.
This would push up the cost of servicing the UK's debts, blowing a hole in the public finances and making it harder to plug the £76billion deficit.
Others to have warned of higher borrowing costs include Wall Street giant JP Morgan, which has donated £500,000 to the Remain campaign, and the world's biggest hedge fund manager - another US firm - BlackRock.