DAVID Cameron has insisted ensuring English MPs have the final say over income tax and other key policies is "vital" to hold the United Kingdom together.

The Prime Minister said there was a "basic constitutional unfairness" about Scottish politicians being able to decide on issues that did not affect their constituents.

He admitted changing the rules to introduce "English votes for English laws" was not an easy thing to do but said the "real threat" was not acting as more powers were devolved to Holyrood.

The comments came as Mr Cameron launched the Tories' English manifesto at an event in Lincoln alongside Cabinet colleague William Hague.

The premier said: "Soon, the Scottish Parliament will be voting to set its own levels of income tax - and rightly so - but that has clear implications.

"English MPs will be unable to vote on the income tax paid by people in Aberdeen and Edinburgh while Scottish MPs are able to vote on the tax you pay in Birmingham or Canterbury or Leeds.

"It is simply unfair. And with English votes for English laws we will put it right."

Mr Cameron said Mr Hague had worked "incredibly hard" to find a solution that did not create two classes of MP and was not "some purist, over-neat solution that ends up creating more problems in the long-run".

"English votes for English laws is not about fragmenting the UK," he added. "It's not about division and difference and pulling apart, it is about making our United Kingdom stronger.

"Because if you have basic constitutional unfairness like we've had, if you have the people in one part of the UK feeling like they are getting a raw deal, then resentment festers and that undermines the bonds and the fellow-feeling that are the basis of the United Kingdom.

"It's not always easy to broach these questions, but long-term it is the right thing to do.

"Because I want our United Kingdom to stick together, to keep taking on the world together, and English votes for English laws is a vital part of that."

Mr Hague said the proposals meant English MPs would have an "effective veto" on measures that only affect their constituents.

"The current situation is manifestly unfair, undemocratic and unsustainable," he said.

Scottish Labour leader Jim Murphy said: "The Tory Party manifesto revealed their commitment to breaking up UK income tax. This is an absolute breach of the agreement they made in the Smith Commission. It is the Tories' own plan for full fiscal autonomy for England.

"We now have a Tory Party committed to an England-only tax system and the SNP hell-bent on Scotland-only taxes. Both outcomes would be bad for Scotland."

SNP depute leader Stewart Hosie said: "David Cameron's plans as outlined today in his party's English manifesto are in stark contrast with what he said during the referendum campaign last year.

"Then he said 'Scotland's got a right to make its voice heard in the UK, Scotland should seek to lead the UK, not leave the UK' . Today he wants to deny Scotland a vote at Westminster on issues where there is a clear Scottish interest. And he has dumped both the spirit and letter of the Smith Commission - which his own party in Scotland signed up to.

"Tory colleagues of Mr Cameron, such as Environment Minister Liz Truss, are making it clear that they want English MPs to have a veto at Westminster over issues such as health spending - despite that having a direct knock-on effect on Scotland's budget."

Mr Cameron later visited the Flakt Woods fan factory in Colchester, before heading to Clacton - which the Tories are hoping to reclaim from Douglas Carswell, who defected to Ukip.

After a speech to activists at Tendring Technology College, Mr Cameron was asked if he really believed they could win the seat back.

"I absolutely do and that is why I am here," he said.

The Tory leader argued that the election was increasingly coming down to a "choice" between him and Ed Miliband.

Banging the lectern, he said he would be "giving it everything I've got for the next 13 days".

Mr Cameron also revealed that the last time he went to Frinton-on-Sea he was chucked by a girlfriend. "I worked out on the bus today I haven't been in Frinton for 33 years.

"I remember it well, I was staying with my girlfriend's family at the time, I had had a lovely game of tennis, I had been for a swim - I thought, how perfect life could be in Frinton," he said. "And she dumped me."