The site, a field off Birdham Road to the southwest of the West Sussex city within the Chichester Harbour Area of Outstanding Natural Beauty, has been occupied by several gypsy families since 2015, the court heard.
It has been divided into individual mobile home and caravan pitches owned by residents, and infrastructure has been installed, including essential services, stable buildings and dog kennels.
Since the development began, Chichester Council has issued no less than four enforcement notices in respect of the site, none of which have been obeyed.
There was a seven-day public inquiry before a planning inspector in May 2017, but residents' arguments that they should be permitted to remain on the site were in every respect rejected. The inspector directed them to vacate and restore the field by November 2018, but that deadline came and went without effect.
That prompted the council to seek a High Court injunction on the basis that all other means of enforcing clearance of the site had been exhausted.
Ruling on the application, Judge Simon Auerbach said: "There was, and really could be, no dispute that the development which has taken place and still exists today on the site, by way of building and similar works that have taken place, and the domestic use to which it is being put, is in plain breach of planning control."
Residents had "wholly failed" to comply either with the enforcement notices or the deadline set by the inspector, and the judge added: "This is a case in which it has been shown that nothing short of an injunction will provide effective restraint of what amount to serious, longstanding and ongoing breaches of planning control."
Against that were the residents' arguments that they had nowhere else to go and that an injunction would put them back on the road. There was no dispute that the two permanent traveller sites in West Sussex, and the one transit site, are all full.
There were three pregnant women living on the site, together with a number of school-age children, one of them suffering from serious health difficulties. Families had made their homes on the site, living there for years, rather than weeks or months.
The judge said that, despite all those factors, the council had put forward a "compelling case" that the "extensive, pervasive and long-standing" breaches of planning control justified the grant of an injunction.
Residents, however, also argued that the court should stay its hand due to the Covid-19 crisis. They argued the pandemic makes life particularly hard for gypsies on the road, restricting their access to washing and toilet facilities.
Track and trace measures "would not work for them" and healthcare would be harder to access. Fear of the virus had bred "greater hostility" against travellers and it was argued that forcing residents off the site to face an uncertain future would amount to a "discriminatory act".
Ruling on the issue, Judge Auerbach said: "No reader of this decision needs to be told by me how extraordinary and pervasive an event the Covid-19 pandemic is, in terms of its health, social and other impacts, in both gravity and scale."
However, the lockdown restrictions imposed by the government on 23 March had since been substantially eased. Although there had been resurgences of the virus and local spikes, none was expected in the Chichester area.
The judge ruled: "After careful consideration, I am not persuaded that the current situation in relation to the pandemic tips the balance against the grant of injunctive relief that would require (residents) to leave within the timescale of the end of the year."
He added: "I consider that the risk of a second wave, or, for example, a local outbreak, can be managed and addressed, if need be, should they arise."
In the event of a local spike, the council could be expected to act responsibly and it would be open to residents to seek a variation of the injunction.
Judge Auerbach ordered residents to cease domestic use of the site by 31 December 2020 and to fully restore the field to its former grassland state by 31 January 2021.