A senior Cabinet Minister has suggested that the Conservatives could scrap their pledge not to raise income tax, VAT or national insurance; a move that would leave the door open to tax rises if the party won the General Election.

Sir Michael Fallon, the Defence Secretary, insisted the Tories were the party of "lower taxes" but admitted it did not want to commit to too many "prescriptive" targets in its manifesto, indicating that the "tax lock" pledge from the 2015 election could be dropped.

Last month, Philip Hammond, the Chancellor, was forced into a humiliating U-turn shortly after his first Budget when a revolt from backbench Tory MPs forced him to ditch planned National Insurance changes for the self-employed.

Theresa May has so far refused to repeat the "tax lock" pledge while Mr Hammond said he needed more "flexibility" in managing the economy, prompting Labour to claim the Conservatives were planning a "tax bombshell" should they retain power on June 9.

Sir Michael said a review of the rights of self-employed workers would continue if the Tories won the election and echoed the Chancellor's call for flexibility.

He told ITV's Good Morning Britain: "What he said is he doesn't want too many targets inside the manifesto that are too prescriptive, that don't allow you, as the situation develops over the lifetime of the Parliament…the flexibility."

Asked if the Tories would recommit to not raising income tax, VAT or national insurance, the Scot said: "The manifesto will certainly make clear which side of this argument we're on; it's Labour governments that increase tax, it's Conservative governments that take the lower paid, in particular, out of tax."

Meantime, the Defence Secretary said the Tories were not planning to freeze energy prices, pointing out how a freeze was "very different" to a cap and that the party would set out detailed proposals on the energy market later in the election campaign.

He told BBC Radio Four's Today programme: "You'll find when the manifesto is published in a couple of weeks we are not proposing a freeze. A freeze is very different to a cap."

Sir Michael denied the Government was being hypocritical after it attacked former Labour leader Ed Miliband's policy of energy price controls in the 2015 poll.

"We didn't introduce a freeze because a freeze would have stopped you taking advantage of prices when they dropped," he declared.

Mr Miliband tweeted in response: "Fallon, of course, talking garbage...2015 manifesto: Labour will freeze energy bills until 2017, ensuring that bills can fall but not rise."

Damian Green, the Work and Pensions Secretary, has said the Tory manifesto will include proposals to cap energy bills.

Speculation is rife that the Conservatives will try to reduce energy costs by around £100-a-year for an average family by capping gas and electricity bills for households paying standard variable tariffs.