Poster claiming Conservatives have halved the deficit is called into question as parties prepare for long battle to win over voters

David Cameron unveiled the Conservatives’ first poster for the 2015 election campaign on Friday with the slogan: “Let’s stay on the road to a stronger economy,” but faced criticism over its claims about his government’s economic record.

The prime minister, visiting Halifax, West Yorkshire, said it was crucial his party won “the most important election in a generation” to safeguard the economy, saying: “Yes, we have cut the deficit in half as a share of our economy but, in the next parliament – the next five years – I want that deficit eradicated completely.”

Alternatives to the Conservatives’ plans would be disastrous, Cameron added.

Critics of the Tory advertisement included Fraser Nelson, editor of the right-leaning Spectator magazine, who said the party should not have started the election battle telling a “porkie” about having halved the deficit.

The shadow minister Chris Bryant, meanwhile, said it was a “bit troubling when the first Tory campaign poster has a fib”.

The poster, due to be on billboards around the country, has slogans superimposed over a long straight road running through countryside, which is blended into a union flag. It lists the party’s achievements as being “1.75 million more people in work”, “760,000 more businesses” and “the deficit halved”.

The last of these claims has caused controversy, because the deficit will only have fallen from its peak of around £153bn to an estimated £91bn by the end of 2014-15.

Despite being challenged about their slogan, Cameron and George Osborne, the chancellor, argue the most natural measure is the reduction of deficit as a proportion of GDP. By this metric, the deficit has fallen from around 10% of GDP to 5% of GDP – as national income has risen.

In a speech launching the campaign, the prime minister was defiant about the poster’s claim, saying his party has “already cut the deficit in half and we have set out clear steps to finish the job by 2018”.

Some voters also mocked the poster for presenting an unreal picture of the countryside and a road with no markings. It is understood the image is actually a computer-generated composite of photographs and not a real place.

One advertising expert, Philip Hesketh, told the Daily Mail: “The irony is, when you look at it a bit closer, the scene looks a bit French. It’s very unusual to have a road that wide with no lines down the middle and stretching so far with no lamp-posts.”

Osborne was also out promoting the poster on Friday in Shropshire and campaigning in Twickenham in south-west London – the seat of the Lib Dem business secretary, Vince Cable. Despite their party political nature, Osborne’s visits were promoted by Treasury press releases boasting of the government’s “long-term economic plan” – a Conservative slogan.

With just over four months to go before the election, the leaders of the two main parties are expected to start spending more time talking to voters instead of working at Westminster. Both parties have established their “narratives” for the election, with the Conservatives focusing on their economic record, and Labour promising to address the cost of living and reduce inequality. Cameron will also have to navigate the issues of immigration and Europe in the face of the threat from Ukip. He will hold talks this week with Angela Merkel, the German chancellor, who is visiting London, and he will be expected to raise the issue of tighter curbs on EU immigration.

But in a further bad start to the year for the prime minister, the former Tory cabinet minister Lord Tebbit warned Cameron he would struggle to win the election unless Ukip somehow “implode”.