Roger Bryant, 73, of Barnstaple, was convicted of 28 offences following a lengthy trial at Bristol Crown Court.

His sons James Bryant, 49,and Scott Bryant, 38, were also found guilty of two charges in relation to them knowing – or at least suspecting – the money generated for their own companies by their father was the proceeds of crime.

Roger Bryant was sentenced to six and a half years in prison, James four years in and Scott two years suspended for 18 months.

His Honour Judge Burgess KC described Roger Bryant’s criminality as “audacious and protracted”, adding that he had a position of considerable trust within The Trust and was responsible for a budget of more than half a million pounds per year.

Judge Burgess said that Roger Bryant actively abused his position and that his sons were consciously complicit in the offending.

During the trial, a jury heard how Roger Bryant was employed by the National Trust as a building surveyor. Whilst in this role he fraudulently submitted invoices to the Trust for building work on National Trust properties that was not completed, either in whole or in part.

Where some work did take place, it was not the work that had been specified on the schedule of works and was not of the standard expected by the National Trust.

Between January 2008 and October 2013 the National Trust believed it was paying two companies for work that had been completed. JR Contracting in most instances and SB Construction in others. During this time period the Trust made 112 payments totalling £1,061,096.29 in respect of invoices from JR Contracting.

The payments were made to one of two bank accounts, both controlled by James Bryant.

Between February 2011 and October 2013 the Trust made 36 payments totalling £64,208.57 after invoices from SB Construction.

The fraud was discovered in October 2013 when the National Trust was seeking to update its approach to procurement. In a meeting with managers for the Arlington and North Devon area, JR Contracting was identified as the biggest supplier to the Trust in that area.

Neither of the general managers for either area had heard of them. When confronted, Roger Bryant said that JR Contracting existed and had done the work.

Following the discovery of the fraud, Roger Bryant attempted to pervert the course of justice by asking two people to lie and say they had witnessed works take place at various locations, when in fact they had not.

In a separate fraud against the National Trust in 2013, Roger Bryant submitted invoices in relation to a real contractor for work supposedly carried out on National Trust properties when the work had been carried out on Roger Bryant’s own property.

Following the sentencing, Detective Inspector Adam Bond said: “This has been a complex case hindered by the passage of time but due to the support and assistance given by representatives of the National Trust, the CPS and prosecuting counsel, the jury who sat through nearly three months of evidence were able to pass guilty verdicts on all but three of the charges leading to today’s sentences.

“I would especially like to thank the witnesses in this case who have persevered throughout what has been a very lengthy investigation.

“The sentences passed today by His Honour Judge Burgess KC reflect the severity of this fraud, especially when considered the victim is a much-loved and respected charitable organisation responsible for much of the country’s building and countryside heritage.”