Rapists and other serious sex offenders may be forced to take lie detector tests when they leave prison, to reduce the chance of them of re-offending.

Downing Street is pressing for the introduction of mandatory polygraph testing, which monitors heart rate, brain activity and blood pressure, across England and Wales. The tests would apply to the 750 most serious sex offenders currently out on licence. A Downing Street source said two pilot programmes in the East and West Midlands probation regions between April 2009 and October 2011 found that mandatory lie detector tests prompted sex offenders to:

• Be more honest with their offender managers. A No 10 source said they provided probation staff with more information about the potential risks they pose.

• Make twice as many disclosures to probation staff, such as admitting that they had contacted a victim.

• Admit the tests helped them manage their own behaviour more effectively.

Any offenders found to have broken their licence as a result of a lie detector test would be sent back to prison. Strict conditions would still apply once they leave prison. They have to sign the sex offenders register and can be banned from certain areas and from contact with named people. There are around 750 serious sex offenders at any one time living in the community on licence.

A No 10 source said that the plan was still in its early stages. But David Cameron is said to have been impressed with the results of the pilot tests and has asked for the costs of extending the scheme across England and Wales to be assessed.

The No 10 source said: "It's vital that we protect the public from serious sex offenders. That's why the conditions after they leave prison need to be both strict and rigorously enforced.

"The pilot schemes using lie detectors to manage offenders in the community have been a success. So now we're looking at how it could be rolled out to provide probation officers with more information to manage the most serious offenders." Hertfordshire police said that last November the force tested "low level" sex offenders, and many presented a higher risk to children than previous estimates. Of 15 offenders tested, eight failed and six passed. One was caught trying to cheat by breathing erratically and talking slowly.

Downing Street is confident that extending the lie detector tests across England and Wales will not leave the government vulnerable to a challenge at the European court of human rights. When an offender took a case to Strasbourg under article 8 of the human rights convention the lie detector was deemed a proportionate way of preventing crime.