The SAS is facing a recruitment crisis because soldiers do not have time to train for demanding selection tests, the head of the infantry has warned.
Brigadier Richard Dennis said it needed better quality applicants, in a letter leaked to the Daily Telegraph.
But Brig Dennis said high tempo and "unrelentingly demanding" Afghan operations were combining to "mitigate against special forces recruitment".
The Ministry of Defence said it did not comment on special forces matters.
Brig Dennis said the SAS was also losing its unique status among the services because "interesting operations are no longer seen as the preserve of special forces".
In the letter to the head of the Army, Gen Sir Peter Wall, he said he had deep concerns over the "challenge of fully manning the SAS" and urgent action was needed to improve the "depth and quality" of potential recruits.
He quoted the commander of 22 Special Air Service Regiment as saying there was an "understandable need for more youthful, quality volunteers".
Brig Dennis said: "I am content, notwithstanding the need to avoid any complacency, that the infantry community delivers sufficient officer and soldier volunteers to selection.
"I am less confident about how we guarantee better depth of quality to increase selection pass rates.
"Indeed, for any measure to be successful you might consider that Army action is essential if we are to increase selection success and the enduring quality of our SF [special forces] community."
BBC defence correspondent Jonathan Beale says there are concerns about the future of the SAS which is understood to be already understrength.
Those worries will have been exacerbated by cuts to the Army, as well as the other services, announced in the government's defence review, our correspondent says.
That means the SAS will have a smaller pool of soldiers from which to recruit.
Our correspondent said the government announced last year that it was increasing funding for Britain's special forces.
Former SAS soldier Robin Horsfall told the BBC joining the elite service no longer had the same appeal.
He said: "Soldiers are seeing active service, all of them are seeing active service, for long protracted periods of time.
"Having been on the battlefront it's less alluring for them to want to go into a more dangerous situation with special forces because they know what battle really is."
Conservative MP Patrick Mercer said: "With the introduction of the Special Reconnaissance Regiment there are now more people fishing from the same pool."
He suggested one of the things which might improve the calibre of the special forces was an increase in pay.
Mr Mercer, a former infantry officer, said it would be a "dire blow" if the quality of British special forces were to fall and their reputation to be damaged.
A Ministry of Defence spokesman said: "We do not normally comment on special forces matters and we can see no reason to change that policy on this occasion."