A Liberal Democrat government will "revolutionise" mental health care in England over the next parliament with £3.5bn ($5.1bn) worth of funding, Nick Clegg has announced.

The deputy prime minister also promised to put mental health "front and centre" of the political debate in the run-up to the general election.

"Only the Liberal Democrats can keep Britain on track and provide both a stronger economy and a fairer society with strong public services," Clegg argued.

"Only the Liberal Democrats can and will make sure mental health is treated with the same urgency as physical health, with money to back that up, and challenge the stigma every day."

The leader explained that the five years' worth of funding will help, among of things, pregnant women and mums dealing with depression and a new waiting time standards for "people in crisis" will be created.

The Liberal Democrat's "Manifesto for the Mind" comes as the Tories promise to create two million additional jobs over the next parliament.

The pledges are part of the general election campaign after David Cameron asked the Queen to dissolve parliament yesterday.

Elsewhere, Ukip leader Nigel Farage is expected to attack Cameron over his failed pledge to reduce net migration to the UK to "hundreds of thousands" by May 2015.

The latest opinion poll from YouGov, which was conducted on 30 March, showed that Labour and the Tories are both on 35%, Ukip on 12%, the Liberal Democrats on 8% and the Greens on 5%.

The research comes after a separate survey, conducted by ComRes between 28 and 29 March, found that the Tories had a four-point lead (36% vs 32%) over Labour.