Britain leaving the EU would damage Western security and dangerously weaken Europe, five ex-NATO chiefs and a host of former US secretaries of state and defence warned on Tuesday.

The former NATO heads said a so-called Brexit would undermine the transatlantic military alliance and "give succour to the West's enemies", and would "undoubtably lead to a loss of British influence".

In a letter to The Daily Telegraph newspaper, they said they were following the debate ahead of the June 23 referendum on Britain's EU membership "with growing concern".

"Given the scale and range of challenges to peace and stability we collectively face, the Euro-Atlantic community needs an active and engaged United Kingdom.

"Brexit would undoubtedly lead to a loss of British influence, undermine NATO and give succour to the West's enemies just when we need to stand shoulder to shoulder across the Euro-Atlantic community against common threats, including on our doorstep."

The signatories were Peter Carington, Javier Solana, George Robertson, Jaap de Hoop Scheffer and Anders Fogh Rasmussen.

In a letter to The Times newspaper, 13 former US secretaries of state and defence and national security advisors said Europe would be "dangerously weakened" if Britain pulled out of the EU.

"The world needs a strong and united Europe to work with the US," said the signatories, drawn from every White House administration over the last 40 years.

"We are concerned that should the UK choose to leave the European Union, the UK's place and influence in the world would be diminished and Europe would be dangerously weakened.

"The special relationship between our countries would not compensate for the loss of influence and clout that the UK would suffer if it was no longer part of the EU."

Signatories included former secretaries of state George Shultz and Madeleine Albright and former defence secretaries Frank Carlucci, William Perry, Bill Cohen, Bob Gates and Leon Panetta.

On a visit to London last month, US President Barack Obama said Britain being in the EU magnified its global influence, and outside the bloc it would go to the "back of the queue" when it came to signing trade deals.

Former British defence minister Liam Fox, who is campaigning for Britain to leave the EU, said the warnings were out of date.

"A lot of those people are from a very different era to the one we are in now," he told BBC television.