The scheme aims to recognising excellent London staff. But the Government has said it will only fund the one-off bonus for the next two financial years.
Now the National Union of Teachers is advising members to register by the end of the month for the final round of funding, or risk losing out.
Some 10,500 of London's 60,000 teachers have registered, but the number may have been inflated because heads are getting as many as possible to register even if they do not want the status.
For each registration, the school gets pound;1,000 to spend on anything, which does not have to be returned if a teacher fails to qualify.
Applications are based on existing performance management evidence but some heads say staff still believe it is too much trouble for pound;1,000.
Angeles Walford, head of Priory C of E primary in Wimbledon, said: "My teachers are not taking it up because they are not planning to be here in the time it will take to qualify, because they can't afford to stay in London."
To get the status staff must meet 12 standards in areas including pedagogy, subject knowledge, and knowledge of London's diverse communities.
It will normally take newly qualified teachers five years to acquire the status. Experienced staff on the upper pay scale should take just two years.
The Government agreed to pay schools extra until 20078 to reflect the large numbers in those signing up to the scheme, which began in 200405.
After that schools must pay bonuses themselves, out of training budgets.