David Cameron has claimed the Conservatives can defeat the UKIP challenge of defector Douglas Carswell in the Clacton by-election.

In a visit to the constituency today, the Prime Minister admitted the Tories will be the “underdogs” when voters go to the polls next Thursday but backed former comedy actor Giles Watling to mount a strong challenge and win over voters considering their allegiance to the former rebel MP.

Mr Watling was selected to represent the Conservatives in Clacton after Mr Carswell stunned Westminster by quitting the Tories and triggering a by-election last month.

When asked whether the Conservatives could overturn Mr Carswell’s large majority, Mr Cameron said: “Of course if we get the votes we can win here.

“Obviously we are the underdogs but we are fighting a very hard campaign because I want Clacton to have a strong Conservative Member of Parliament who can help deliver our long-term economic plan.

“Giles is an excellent candidate. He is a really first-class, hard-working, local Conservative who has already done a lot for the district.”

His intervention came during a visit to the £36 million sea defence scheme, funded by local and national government, in Holland-on-Sea today.

Mr Carswell said he defected to UKIP after becoming disillusioned with Conservative policies on Europe, migration and the deficit reduction.

But Mr Cameron argued Mr Carswell “let people down” when joining Nigel Farage’s party.

The Prime Minister said: “It is totally counter-productive because he (Mr Carswell) says he wants an in/out referendum on Europe, and the only way you can get that is by having a Conservative government.

“We are putting a Conservative manifesto in place and we are investing in Clacton. You can see it in the flood defences, you can see it with the work we want to do in Jaywick. There is a lot of regeneration work going on – but UKIP are not interested in regeneration.

“All Douglas Carswell will be is a notch on Farage’s bedpost, and more to the point, a notch on Ed Miliband’s bedpost – and that is in nobody’s interest.”

Meanwhile, Mr Watling, who toured the sea defence scheme with Mr Cameron, issued an upbeat assessment of his chances of winning the by-election.

He said: “We always knew it was going to be a challenge from day one. We always knew it was going to be a tight race. It is still going to be a tight race but I feel things are moving in my direction. To begin with, we started right on the back foot because we were not expecting what happened. But I am really delighted to say that on the doorstep now, I am getting people moving towards me. They are even changing their minds as I speak to them.

“I think it (loyalty towards Carswell) is dissipating a bit. Until three weeks ago I was partly responsible for running Douglas Carswell’s campaigns so I was intimately involved in making him the acceptable face of Conservatism in this area.

“With Douglas doing what he did, I now find myself in the position of opposing him and actually, as I’m going on the doorstep, I’m finding it easier and easier to talk to people about Douglas and they are backing away from him.

“People not liking what Douglas did is a big response I am getting and people are uncertain of UKIP. Nobody knows what it represents. It’s like trying to put your finger on a piece of mercury: you have no idea. Every time you think you have got a policy – off it goes in another direction.”

Standing as a Tory in 2010, Mr Carswell gained a majority of 12,068, or 28%. But Mr Watling, who grew up in Clacton, said he would deliver a package of improvements to the town if selected.

Mr Watling said: “We have got 3,500 more jobs in the area since the last General Election – and we are not going to sit on our laurels about that.

“I want to improve the tourism offer, I want to keep business rates low – and I will lobby to do that.

“I had Patrick McLoughlin (Transport Secretary) down just recently and I was lobbying him about improving the rail services. We are just an hour from one of the major conurbations of western Europe and we need to exploit that and get people down here to see how wonderful it is and we can get this place really moving.”

The sea defence scheme in Holland-on-Sea is one of the biggest of its kind, stretching over 5km and involving the construction of 100 groynes to protect homes and businesses.