Cameron embarks on marathon tour of Britain in last-ditch bid for votes

Kenneth Clarke admitted the Conservatives had only a "slim" chance of securing an overall majority in tomorrow's general election as the party launched a desperate last-minute effort to pass the winning post.

With time running out to capture millions of wavering voters, David Cameron dispensed with sleep and campaigned across the North of England through the night.

Tensions are running high within Conservative high command as polling approaches despite recent signs that the party's opinion poll rating is hardening. Although Tory strategists insist they are receiving better feedback on the doorsteps than the polls suggests, one source said: "We're not there yet, but still hope to be by Thursday."

The party's private anxieties were publicly confirmed by Mr Clarke in an interview with the website politics.co.uk. The Shadow Business Secretary said: "I think there's a still a slim chance we can get an overall majority, which I would very much like to see. It is very difficult to read because the electoral geography is quite local and it is a complicated breakdown of voting which is taking place. But my hunch is we're still in with a chance of getting a parliamentary majority."

Mr Cameron was due to meet night-shift workers – including firefighters, bakers and florists – in Carlisle, Darwen and Wakefield before heading to Grimsby this morning to greet the town's fishermen returning to port. In a final sweep through marginal seats today, the Tory leader will head for the East and West Midlands and Wales, before closing his campaign with a rally in the South-West.

The punishing schedule was designed to reinforce the message that Mr Cameron has the energy and drive to win the most unpredictable election in a generation.

A nine-minute video was emailed yesterday to 500,000 people comparing New Labour's ambitions upon taking office 13 years ago with its record. It highlights knife crime, child poverty, the recession, national debt, sleaze and government waste.

Yesterday Mr Cameron flew into Belfast to energise his Ulster Unionist sister party with assurances that a Tory government would not target Northern Ireland's extensive public sector for particularly tough cuts.

Mr Cameron, who arrived late because of flight disruptions caused by the volcanic ash cloud, repeatedly stressed his commitment to the union between Britain and Northern Ireland. He declared: "We are showing that we are the party of the union. I will never be neutral on the union." He received resounding cheers from a unionist audience when he vowed: "There is no way Northern Ireland will be singled out over and above any other part of the UK."

The Tory leader promised to study ways of turning Northern Ireland into a enterprise zone, of attracting more investment and of helping investors who lost investment cash in a Presbyterian society.

His sentiments will cause murmurs of anxiety in non-unionist circles, since his words might be taken as signalling that a Conservative government would not seek to observe equality between unionist and nationalist traditions.

Mr Cameron's declaration that the Tory-Ulster Unionist partnership has created "a dynamic new electoral force" has not been borne out by opinion poll evidence, which indicates that Peter Robinson's Democratic Unionists will continue to dominate the unionist landscape.

A Belfast Telegraph poll suggests in fact that the Ulster Unionist party may win none of Northern Ireland's 18 Commons seats. Such a result would obviously be of no help to Mr Cameron in a hung parliament.

He can expect no assistance either from nationalists such as the Social Democratic and Labour Party, which is aligned to the British Labour party.

Sinn Fein, which is expected to hold at least four of its five Commons seats, has a policy of abstentionism stretching back a century or so, its members never taking their seats and never voting.