Labour's Sadiq Khan, the front-runner to be the next mayor of London, will pledge to tackle the capital's NHS "crisis".
The move comes as Ukip unveils its plans for London if the party takes control of City Hall on May 5.
Mr Khan will use a speech to insist the NHS needs a mayor who will go into battle with the Government for medical resources.
"NHS staff are overstretched and overworked," Mr Khan will say. "I'll be the first mayor to provide real leadership for London's NHS.
"The Tories short-sightedly abolished NHS London - meaning London's health and social care system doesn't have the strategic planning and co-ordination it needs. As mayor, I will fill that vacuum.
"London's NHS is in real crisis under the Tories. Waiting times in A&Es are rising, with some even turning patients away.
"Ambulance response times are endangering lives. Mental health services are simply unable to cope, and cuts to social care services mean Londoners aren't receiving the early support they need to avoid hospital admissions.
"NHS staff are overstretched and overworked so hospitals are spending huge amounts on temporary staff - as they struggle to recruit and train nurses.
"There has been a 156% rise in the amount London's hospitals are spending on agency nurses in just four years and one in six nursing posts in London is currently vacant," Mr Khan will say.
Ukip is promising a crackdown on crime if its mayoral candidate, Peter Whittle, is elected.
The party will prioritise affordable housing, and lobby the Government to "prevent non-British nationals from accessing right-to-buy, or help-to-buy schemes".
Ukip says it will give Metropolitan Police borough commanders more control over planning and resources.
Its manifesto says the party will "take a zero tolerance approach to 'cultural crimes' including female genital mutilation, of which there were 4,000 recorded instances in London in the last five years and zero successful prosecutions."