William Hogarth works in the oldest hospital in London, which is planned to be renovated | William Hogarth

Two paintings by the author William Hogarth on the walls of the grand staircase at St Bartholomew’s Hospital in London will be restored with the help of a £4.9m grant from the National Lottery Heritage Fund.

Hogarth, an artist, critic and satirist, depicted two biblical stories in works, The Pool of Bethesda and The Good Samaritan, in the 1730s. They feature figures 2.1 meters (7 ft) tall, some drawn from real life.

The paintings will be restored as part of a project to preserve and restore the city’s art collections and historic buildings London hospital known as Bartz. The rooms include a Grade I listed north wing with a great hall and staircase decorated by Hogarth.

The artist, who was born in 1697 in Bartholomew Close, near the hospital, was furious when he learned that the Italian painter Jacopo Amigoni had been invited to paint the staircase. Hogarth stepped in to offer his services for free, even though he had never painted on such a scale.

The Bath of Bethesda depicts a man who had been unable to walk for years, who was healed by Jesus. Hogarth’s painting includes sympathetic portraits of people suffering from illness and injury. Such conditions of hospital patients as gout, rickets and jaundice are recognized by an experienced eye. The painting is still used as a teaching tool.

Hogarth requested that the canvases never be varnished. But when they were cleaned in the 1930s, seven layers of varnish were removed. They were cleaned again in the 1960s.

Special cleaning and conservation is now required to address the structural problems caused by the wooden framework of the “stretcher” behind the canvases.

The staircase hall will also undergo extensive restoration work, including the repair and strengthening of the wooden staircase and the conservation of the 18th century chandelier.

Will Palin, chief executive of Barts Heritage charity, said: “Hogarth’s Staircase is a real ‘hidden treasure’ and is just one element of our ambitious project which combines the much-needed restoration of one of the UK’s most important historic hospital buildings with a pioneering heritage and health programme.”

Eilish McGuinness, chief executive of the National Lottery Heritage Trust, said the paintings were “truly a treasure to be admired”.

Barts is Britain’s oldest hospital, providing free healthcare in one place for almost 900 years.