EU travellers can refuse to hand over their personal information to the government's e-Borders programme, the government has revealed.

Conservative politicians have described the existence of voluntary security screening as "extraordinary" following the revelation.

A letter, published today, from a senior European Commission official, makes it clear that EU citizens cannot be stopped from coming into the UK if they have refused to hand over personal details so that the UK Border Agency can check them against lists of known and suspected criminals and terrorists.

The Home Office published the letter in order to counteract a claim by the home affairs select committee that the e-Borders system would be found to be illegal by the Commission.

The letter from Jonathan Faull, director general of the European Commission's Justice, Freedom and Security department, outlines the 'commitments and assurances' offered by UKBA to make the scheme workable under European law.

'Passengers must be told in advance that handing over the information is neither compulsory nor... a condition of purchase and sale of the ticket,' it states.

'Passengers who are EU citizens or their family members will not be refused entry/exit, or incur sanctions in any way, on the basis that their passenger data is unavailable to the UK authorities for whatever reason,' it continues.

'Carriers will be instructed by the UK authorities not to deny boarding to travellers regardless of their nationality, who do not communicate API (Advanced Passenger Information) to the operator, and that the provision of API data to operators is neither compulsory nor is made a condition of purchase and sale of the ticket.'

Currently, 40 per cent of journeys are covered by the scheme, and the government hopes to increase that to 95 per cent. It also hopes to use the e-Borders system to count the number of people going in and out of the country.

Tory immigration spokesman Damian Green said: "It seems extraordinary that, this long into a seven-year contract that costs more than £1bn, the government hasn't established whether it can impose this system on travellers, and it looks from this letter as if it cannot.

"This is a huge embarrassment for ministers. They have set up this elaborate, hugely expensive system and the Commission is telling them it only works if people volunteer."