Mr Cameron and Mr Davis will visit a school in Hull and talk to sixth-form students about the civil liberties issues which the shadow home secretary is hoping to make the focus of the July 10 vote.

Both men have previously dismissed press reports of tensions over Mr Davis's decision to quit his seat and fight a by-election in protest at the Government's plans to introduce 42-day detention before charge for terror suspects.

Labour accused Mr Cameron of seeking to make the police fight crime "with one hand tied behind their back", because of his support for Mr Davis's campaign against the increased use of CCTV and the national DNA database.

Neither Labour nor the Liberal Democrats are putting forward a candidate in the by-election, where Mr Davis faces opposition from 25 other candidates, ranging from the Green Party to conspiracy theorist David Icke, Miss Great Britain Gemma Garrett, the Official Monster Raving Loony Party and a representative of Make Politicians History.

Rape law campaigner Jill Saward launched her campaign manifesto with a promise to reform the criminal justice system to "put victims first".

Ms Saward, who was attacked in the infamous Ealing Vicarage rape in 1986 and later waived her right to anonymity in order to campaign for the rights of sex attack victims, said Mr Davis "would achieve nothing by winning the election".

She said there needed to be "balance" in the criminal justice system, claiming the "pendulum has swung too far in the favour of the wrong-doer".

Labour's Home Office minister Tony McNulty said: "As David Cameron today joins David Davis's campaign against our policies on terrorism, CCTV and DNA evidence, I challenge him to explain to the victims of crime why he wants to make it harder for the police to put criminals behind bars.

"Without our policies on DNA and CCTV the police would find it harder to catch rapists and murderers."