The matter was first brought to her attention by guests who spotted it from Royal Windsor Way – a dual carriageway that brings them into the historic town.

A royal source said: "The Queen was extremely upset to hear that this view of Windsor Castle has been turned into such an eyesore.

"Her aides have been asked to see what can be done to have this gratuitous vandalism cleaned up and the views across to Windsor Castle restored to their former beauty."

The graffiti artist is unknown but the tag HELCH has been scrawled on many other sites in and around London including many bridges on the M4, M1 and M25.

It has also been emblazoned on London railway and tube stations and disused buildings.

'EXTREMELY UPSET'

West Windsor Residents Association chairman Richard Endacott said: "The Royal Windsor Way is the gateway into our town and is the first view that you get of Windsor Castle.

"For this piece of vandalism to ruin this view is just devastating and I am sure the Queen will not be impressed and it is essential that action is taken to restore a one fantastic view.

"However much it costs we have 1.4m tourists visiting Windsor Castle each this year and this should not be their first sighting of it and the sort of memory they take away of it.

"When the person who did it caught he should have to pay for the cost of cleaning it up. It is illegal to graffiti like this and it is a criminal offence and it blights our town."

Local Susan Roberts said: "It is quite incredible that someone can paint something as big as this as it must have taken several nights without anyone apparently noticing anything."

'BLIGHTS OUR TOWN'

The three-mile single track railway between Windsor and Slough opened in 1849 and the original wooden trestles were replaced by a 2,035-yard brick viaduct in 1865.

The line was used by Queen Victoria’s royal steam train and the famous playing fields of Eton College where Prince Harry and William went to school lay on the other side of it.

The viaduct takes the railway over the River Thames and is the world’s only wrought iron bridge still in regular use. It was designed by Isambard Brunel and is Grade 2 listed.

The viaduct has been the target of graffiti in the past but Network Rail, which is responsible for its upkeep, has said it will only remove it if it is obscene or offensive.

A spokesman said: "All graffiti removal comes at the taxpayers’ expense and we only remove it when it is deemed offensive or else the people who do it go back and just do it again."