William Henry Francis, 11th Baron Petre (22 January 1793 – 3 July 1850) was an English nobleman, based in Essex. He was the first Baron Petre to take his seat in the House of Lords after the passing of the Roman Catholic Relief Act 1829.

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He was a son of Robert Edward Petre, 10th Baron Petre of Ingatestone Hall and Mary Bridget Howard. His mother was a sister of Bernard Howard, 12th Duke of Norfolk.

Petre was a passionate horseman and maintained, and was master of, his own pack of foxhounds (1822–1839) known as the "Thorndon Hunt", from which the Essex Union Hunt subsequently developed. He also constructed a racecourse at Oxney Green, near Writtle. It is said that following the Battle of Waterloo, Petre acquired Marengo, the grey Arabian horse of Napoleon I of France

Keen on hunting he also created, at Thorndon Park, a mixed herd of up to 2,000 fallow deer and red deer which subsequently formed the basis not only of the present Brentwood, Essex herd but also the entire deer population of New Zealand.

On 2 June 1815, Petre married his first wife Frances Charlotte Bedingfeld (19 April 1796 – 29 January 1822 Thorndon Hall), daughter of Sir Richard Bedingfeld, 5th Baronet and his wife Charlotte Georgiana Jerningham. Her maternal grandparents were Sir William Jerningham, 6th Baronet and Frances Dillon. The senior Frances was a daughter of Henry Dillon, 11th Viscount Dillon and Lady Charlotte Lee. The senior Charlotte was a daughter of George Lee, 2nd Earl of Lichfield and Frances Hales. The eldest Frances was a daughter of Sir John Hales, 4th Baronet.

Frances, nicknamed "Fanny", became a letter writer of brilliant vividness. They had four children. Frances died in childbirth.

• William Bernard Petre, 12th Baron Petre (20 December 1817 – 4 July 1884).
• Mary Agness Petre (1818 – 22 August 1886). Married James Alexander Douglas.
• Henry William Petre^[1] (23 January 1820 – 3 December 1889). Member of the New Zealand Legislative Council^[2] and father of Francis Petre.^[3]
• Charlotte Elizabeth Petre^[1] (29 January 1822 – 31 October 1903). Married her first cousin, Captain Charles Edward Petre. He was a son of Charles Berney Petre, a younger brother of the 11th Baron.

On 14 April 1823, Petre married his second wife Emma Agnes Howard (5 November 1803 – 10 February 1861 London). She was a daughter of Henry Howard of Corby Castle and his second wife Catherine Mary Neave.^[4] Her paternal grandparents were Philip Howard and Ann Witham. Philip was a son of Thomas Howard and Barbara Musgrave. Thomas was a son of William Howard and Jane Dalston. William was a son of Sir Francis Howard and Margeret Preston.

Sir Francis was a son of Lord William Howard and Elizabeth Dacre. His paternal grandparents were Thomas Howard, 4th Duke of Norfolk and his second wife Margaret Audley. His maternal grandparents were Thomas Dacre, 4th Baron Dacre and Elizabeth Leyburne.

Petre and Emma had eight children, including;

• Frederick Charles Edmund Petre^[1] (22 December 1824 – 18 June 1906).
• Agnes Catherine Louisa Petre (29 January 1826 – 25 May 1891). Married Charles Clifford, 8th Baron Clifford of Chudleigh.
• Arthur Charles Augustus Petre^[1] (29 March 1827 – 4 November 1882). Married Lady Katherine Howard, a daughter of William Howard, 4th Earl of Wicklow.
• Edmund George Petre^[1] (23 June 1829 – 1 September 1889), father of Francis Loraine Petre, the military historian.
• Albert Henry Petre^[1] (15 March 1832 – 5 April 1917). Married Katherine Elsie Clark, a daughter of William Robinson Clark.

It was during William's lifetime that the railways developed and he proved a stubborn negotiator in his dealings with the Eastern Counties Railway over the Brentwood to Chelmsford stretch of line. During the 1830s, plans were drawn up for the construction of a railway line eastwards from London through Essex. The new railway proposals required Parliamentary approval, but encountered strong objections, and claims for compensation, from Lord Petre, a major landowner along the projected route.

In the House of Lords, he resisted the Act of Parliament the Company needed to precede until they agreed to pay him six times the compensation originally offered. Lord Petre feared that the railway would divide his estates, passing as it would through Ingatestone itself. He suggested that the line should pass further northwards, nearer to Writtle. He also expressed concern with regard to the effects of the inevitable influx of railway navvies: the latter, many of them from Ireland, had a fearsome reputation, and Essex, as yet, without a modern police force. A legal dispute ensued, but Lord Petre eventually withdrew his opposition to the planned route. Royal Assent was granted to the Eastern Counties Railway Company's bill.

However, a fresh dispute began as Lord Petre protested at the inadequate sum awarded to him as compensation for the loss of agricultural land. Construction workers were ordered from his estates. Lord Petre sought a sum of £20,000 for the land required by the railway company, and a payment of a further £100,000 in compensation. The Eastern Counties Railway Company protested at the latter claim, insisting that a panel of surveyors had considered £20,000 a fair total (over £1 million today). The dispute persisted, but after a period of deadlock, Lord Petre won his case and the railway company paid him six times the compensation originally offered. The money was invested in an agricultural estate in the Dengie Peninsula but with the onset of the agricultural depression of the 1880s, the investment proved disastrous.

The construction from London eastwards began in 1838, but it became apparent the Charity ground in Stock Lane was endangered, as the railway company would require part of the property for the excavation of a cutting. Some of the buildings would have to be pulled down, the original almshouses were then moved and it was here that the first station at Ingatestone was situated, approached from the road by a flight of steps. The Brentwood to Chelmsford section of the railway was completed by 1841. The line eventually reached Colchester in 1843.

William had 12 children and 56 grandchildren. His two eldest sons alone (William and Henry) had 29 children between them. He was a strong supporter of overseas colonisation – he was wont to say that, with so many children, he could not be otherwise. Frederick and three of his brothers went to the United States while their elder brother, Henry, joined the first expedition of settlers to New Zealand. Henry's son, Francis Petre, was a leading architect who designed the Roman Catholic Cathedral of the Blessed Sacrament (Christchurch, New Zealand).

Frederick Charles Edmund Petre (1824–1906) was William's 3rd son and Agnes Louisa Catherine Petre (1826–1891) who later became Lady Clifford was his third daughter. In addition, by his second wife, he had Edmund George Petre, (1829–1889). In 1845, Mary Agnes Petre (1816 – ) later married to Mr Douglas, took an inventory of the pictures at Thorndon Hall, the book is divided into a number of sections, each executed in a different style of illumination.

His tenth son, Edmund, was a stockbroker and the father of Francis Loraine Petre, an Indian Civil Servant. Upon retirement, F. Loraine Petre wrote several regimental histories, and five books on the Napoleonic Wars.^[5]