The Conservatives have admitted that only a small fraction of British ports would be included in their widely touted plans to conduct round-the-clock surveillance for illegal immigrants.

The party's manifesto says: "We will ensure 24-hour surveillance at our ports."

But Oliver Letwin, the shadow chancellor, conceded that patrols would only be mounted at "35 major ports" of the estimated 650 in the country.

The Public and Commercial Services union, which represents immigration officers, attacked the "tone and nature" of the party's pronouncements on the subject,  claiming that the processing of applications would deteriorate because of threatened Tory cuts to the service.

Meanwhile, the Conservatives have been forced on to the defensive over the continuing row over a candidate who doctored a photograph of himself protesting about the deportation of an asylum-seeker.

Michael Howard has refused to sack Ed Matts, the Tory candidate in the Labour-held marginal seat of Dorset South, for altering the picture of himself with Verah Kachepa.

The health secretary, John Reid, branded  Matts' actions a "sick" political stunt and accused the Tory leader of defending "deceit and duplicity" to exploit immigration as a campaign issue.