About 2,000 dentists walked away from the NHS after not signing the new contract, leaving up to 1m patients without dentists.
Many dentists are unhappy with the new arrangements
Local health bosses have already started filling the gaps by recruiting graduates and dentists from abroad and offering NHS dentists extra patients.
But dentists leaders are warning more may walk away as many signed "in dispute".
In total, nine in 10 dentists signed the contract by the start of April.
The British Dental Association (BDA) said 60% of those who had signed were unhappy with the terms of the contract and could still opt out, although the government said it was half that number.
The NHS Litigation Authority has already received 500 letters from dentists disputing their contract.
But Health Minister Rosie Winterton said: "Claims that dentists would leave the NHS in mass exodus were unfounded."
While it is estimated 10% of dentists refused to sign, these account for just 4% of NHS services as they tended to carry out more private work.
The reforms were introduced in a bid to improve access to services as a tenth of dental practices had closed their lists to new NHS patients, leaving 2m unable to register with a dentist.
In the short-term the new contract has not done anything to remedy this with primary care trusts, responsible for commissioning services, currently trying to fill the gaps created by the walk-out. In some areas this has been achieved.
However, Ms Winterton believes in time access to NHS dentistry will improve as primary care trusts use their new commissioning powers to encourage extra practices to open in under-serviced areas.
Early figures suggest Avon, Gloucestershire, Wiltshire, Hampshire, the Isle of Wight and the West Midlands have been the worst hit by the walk-out.
The failure of a tenth of England's 21,000 dentists to sign the contract is in line with estimates by the BDA.
BDA chairman Lester Ellman said: "We know many have not signed the contract, but what is more many more signed in dispute and they could still walk away.
"There is still much to be resolved, dentists are not happy about this contract."
The reforms were meant to introduce a simpler system of fees and reward dentists for carrying out more preventative work. But dentists have complained the deal did not live up to expectations.
Acting chief dental officer Barry Cockcroft said: "Dentists told us they disliked the previous system where they had to claim for every single item of treatment.
"This outdated way of working contributed to dentists reducing their commitment to the NHS and we have been working to reverse that."
Under the new contract, dentists are being paid a guaranteed income for the next three years for doing 95% of the courses of treatment they have done in the past.
Shadow Health Secretary Andrew Lansley said: "This new contract is a bad deal for dentists and a bad deal for patients."
And Steve Webb, Liberal Democrat health spokesman, added: "The long-term future of NHS dentistry looks bleak unless the government urgently review the new contract."