Some 270 veterans from the Army, Royal Navy and Royal Air Force have issued a last-ditch plea for comrades to register to vote to back Brexit.
A letter, organised by the Veterans for Britain group, expressed concerns about the increasing role of the European Union in defence policy.
In the letter to The Sun, retired sergeants, petty officers and other non-commissioned officers from the three services hailed the referendum as "a unique opportunity to return full democratic control to people the country elects".
"We believe it is essential to maintain and where necessary re-establish the United Kingdom's autonomy in defence in the context of its pre-existing alliances and to ensure it is directly and solely accountable to the UK Parliament," they said.
"The UK Armed Forces perform a distinctive role in underpinning the relationship between the people and UK head of state and elected government and we are concerned that this role and this relationship is being muddied by the increasing role of the EU in UK life, including its powers in areas of defence."
They said Nato had kept the peace in Europe and a post-Brexit UK would continue to work closely with its allies, including those in the EU.
Voters need to register by midnight on Tuesday and the letter appealed to the more than three million veterans to make sure they can have their say.
"You have served your country and we now urge you to do so in this referendum by registering to vote and voting to leave the EU," the letter said.