Poland had already announced that it would make a significant contribution to the Czech-led plan and that it would provide logistical support to help the shells get to the front.
"We have already declared a significant amount, and in the last 48 hours... it has been decided that it will be doubled," Sikorski told a press conference.
He declined to say how much money Poland was contributing.
Earlier in March, Czech President Petr Pavel said the plan to buy 800,000 rounds of ammunition for Ukraine had secured enough funding.
(Reporting by Alan Charlish; Editing by Alex Richardson)
