The Tories have launched a highly personal attack on Jeremy Corbyn, warning the Labour leader would unable to take the "difficult decisions" needed in the event of a major terrorist attack.
Conservative Party chairman Sir Patrick McLoughlin said it was "blindingly obvious" Britain would be safer under Theresa May and that Mr Corbyn was "not suitable" to take on the responsibilities of prime minister.
His comments, in an interview with The Sunday Telegraph, are the first clear sign that the Tories intend to make the Labour leader's fitness for office a key issue in the general election campaign.
They came as The Sunday Times said Mrs May would make a grab for the political centre ground by cutting £100 from energy bills and promising new rights for workers.
The party's manifesto - due to be published on May 8 - is expected to include a cap on gas and electricity bills for the seven out of 10 households on standard variable tariffs.
Meanwhile Liberal Democrat leader Tim Farron insisted that he would not be prepared to enter into a coalition with either the Conservatives or the Labour Party under Mr Corbyn.
"There is no way we can countenance any kind of arrangement or coalition with the Conservative Party and likewise with the Labour Party led by Jeremy Corbyn," he told The Observer.
"He (Mr Corbyn) accepted hard Brexit, he voted for it. He enabled it. It has put us in the situation we are now in."
In his interview, Sir Patrick sought to contrast Mr Corbyn as a "rebel without a cause" in the Labour Party with Mrs May's record in office as home secretary and Prime Minister.
"There are decisions which prime ministers have to take and those people in authority have to take (which) are sometimes very uncomfortable," Sir Patrick said.
"If they don't take them, we're at danger ... I know that with Theresa May, she would take them. I'm not sure that Jeremy Corbyn would."
Asked if he had concerns about Mr Corbyn's record on security and defence, Sir Patrick said: "The man is not suitable to become prime minister of this country. He has been a rebel without a cause in the Labour Party.
"With Theresa May's record as home secretary and the way she's conducted herself since she's been Prime Minister, I don't think anybody can be in doubt as to how serious she takes those responsibilities and is really on top of the detail.
"I just think she has shown herself as the person most qualified and most suitable to the job."
Labour warned the Conservatives' claims on energy bills should be treated with "a pinch of salt" as they had repeatedly broken promises to deal with the issue in the past.
Andrew Gwynne , the party's campaigns and elections chairman, said: "The Tories don't stand for working people, their record is one of failure and broken promises, letting ordinary people down at every turn.
"The Tories' promises to deal with energy bills should be taken with a huge pinch of salt.
"Time and again they've promised action but when it comes to it they broke those promises. Under them, energy bills have soared.
"At the last election, when Labour promised action the Tories opposed it, putting themselves on the side of protecting the big energy companies' profits rather than the interest of working people."