The Conservatives have warned Tony Blair not to use a massive e-mail database collected on the Downing Street petitions website as a way of blitzing voters with Labour's party-political propaganda.

Chris Grayling, the Shadow Transport Secretary, has written to Sir Gus O'Donnell, Britain's top civil servant, after the Prime Minister sent a four-page personal e-mail to all 1.79 million people who signed a petition opposing road-pricing.

Mr Blair's e-mail set out the Government's policy on the matter in detail, and the Tories are worried that Labour is effectively using a taxpayer-funded website to seek party-political gain in a way that other parties are unable to match.

The petitions website, which was set up jointly by Downing Street and the political scrutiny charity mysociety.org, asks signatories to all petitions for their e-mail addresses. According to data protection rules detailed on its website, Downing Street can then contact each person who signed the petition twice, as can the person who set up the petition.

The Tories are worried that the database could be used by the Government as a way of strategically spamming voters with Labour policy - or that it could fall into the hands of the Labour's strategists and campaigners close to election time to help them with strategic canvassing.

In his letter to Sir Gus, who is the Cabinet Secretary, Mr Grayling said: "Clearly, the use of this database in a way that promotes Government policy would give the Labour Party a substantial political advantage, particularly if further use is made of it by Ministers much closer to the potential date of a General Election.

"This has become particularly true given the level of response to the petition on road-pricing."

Mr Grayling demanded that Sir Gus clarify what the Government's "rights of exploitation" are on the database. He also asked what steps the Cabinet Secretary had taken to ensure the database did not fall into the hands of the Labour Party, and demanded to know whether a future Prime Minister could use the database or whether it would be limited to Mr Blair.

"It is clearly essential that taxpayers' money is not used in a way that provides any political party with an electoral advantage," he said.

A Cabinet Office spokeswoman said that Sir Gus was examining the letter, and would investigate.

However, Downing Street rejected Mr Grayling's demand, saying that the database was strictly limited to two e-mails per signatory, both of which had to be related to the content of the petition in question. A spokeswoman added that the database could not fall into the hands of the Labour Party, as it was controlled by mysociety, which strictly regulated it to ensure it conformed to Data Protection rules.

"We do not hold any data. It is protected by mysociety. There is simply no question of this data being misused," she said.

However, despite the guarantees, Conservative bloggers were sceptical that the petitions facility would not be used by the Government for targeted mailing.

An entry on the ToryDiary blog of Conservativehome.com said that - even including the restrictions on the database's usage - it "still gives the Government the potential opportunity to communicate to millions of voters in the run-up to the next General Election on an issue that petitioners are motivated by".

It added: "Will the Transport Secretary or Prime Minister, for example, be able to communicate to all the petitioners on road pricing about the Government's general approach to issues of concern to motorists?"

The issue encouraged a flurry of reader comments, including: one which said: "I thought of this before I signed up to the EU Referendum petition.

"They are sneaky b*stards and I'm sure that they will attempt to use the data."

However the group that organised the petition, the Association of British Drivers, said it did not object to Mr Blair's mass e-mail of this morning. According to the privacy rules on the Downing Street website, the organisation has the right to send all those who signed the petition up to two e-mails each.

Hugh Blaydon, one of the group's founders, told Times Online: "We are all adult enough to hit the delete button if we see something from the Government. That's what I'd do, because I personally don't believe a word they say."

Meanwhile, the Downing Street website today announced it would follow up Mr Blair's road-pricing e-mail by putting up Dr Stephen Ladyman, the transport minister, in a special road-pricing webchat tomorrow.

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