The British foreign secretary and the Archbishop of Westminster have joined forces in opposing the route of Israel's vast barrier along the West Bank, which adversely affects a community of monks, nuns and Christian families near Bethlehem.

In a private letter seen by the Guardian, William Hague told Archbishop Vincent Nichols that he shared his "concerns about the problem of land confiscation by the Israeli authorities affecting the people of Beit Jala and similar Palestinian communities in the occupied territories".

The letter suggested that the religious orders in Beit Jala needed to give a "clear signal" of opposition to the barrier's route to bolster a legal case against the state of Israel. Shortly afterwards, the monks joined the legal challenge. A ruling in the case is expected by the end of this year.

In addition to Hague's personal intervention, the British consulate in East Jerusalem is supporting the community and the Department for International Development (Dfid) is providing indirect funding for the legal challenge.

The consulate is championing the case as a symbolic example of the impact of the separation barrier on Palestinian communities and the loss of Palestinian land. Around 85% of the barrier is inside the West Bank.

British government policy is that Israel is entitled to build a barrier but it should lie on the internationally recognised 1967 Green Line, not on confiscated Palestinian land. It is concerned that the route is harming the prospects of a two-state solution to the Israeli-Palestinian conflict.

Dfid has given a two-year grant of £2.9m to the Norwegian Refugee Council, which in turn is funding the Society of St Yves, a Jerusalem-based Catholic human rights organisation, which is assisting the Beit Jala community with its case.

The Archbishop of Westminster, England's most senior Catholic, espoused the case of the Cremisan monastery and convent, and their surrounding Palestinian Christian community, in Beit Jala following a visit last November. In last year's Christmas Eve homily, two days after the date of Hague's letter, the archbishop offered prayers for the community's "legal battle to protect their land and homes from further expropriation by Israel".

He went on: "Over 50 families face losing their land and their homes as action is taken to complete the separation/security wall across the territory of the district of Bethlehem. We pray for them tonight."

Nichols's principal concern was to ensure that the Christian presence in the Holy Land was preserved, his spokesman told the Guardian. "The numbers are dwindling. The archbishop is trying to encourage and give succour to Christian communities. The imposition of this wall will jeopardise the livelihoods of many of the families."

Nichols was aware that "sensitivities are high," the spokesman added.

Beit Jala has a population of around 10,000, more than 80% of whom are Christian.

Under the current Israeli plan, the barrier will run between the monastery and convent, separating the two establishments and cutting off the monks from the local Christian community. It will also separate the convent and more than 50 families from land they own.

As part of the campaign against the barrier's route, a mass is conducted each Friday by Beit Jala's parish priest, Father Ibrahim Shomali, under olive trees overlooking the Cremisan valley. "This is Palestinian land," he said. "If Israel wants to build a wall, they should put it on their own land."

Many Palestinian Christians have emigrated as a result of the economic impact of the separation barrier which has already been built around the city of Bethlehem and its nearby villages. Residents have difficulty in accessing their land and exporting their produce. The hurdles in reaching Christian holy sites in Jerusalem is another factor encouraging them to leave.

"People are migrating because of the situation. There is no work, and living costs are high," said Samira Qaisieh, 47, whose stone house with views across the valley has belonged to her husband's family since it was built almost 100 years ago. "If the situation stays as it is, I'm also thinking of leaving with my children."

The terraces around Cremisan, on which Palestinian families picnic at weekends under olive and fruit trees, are overlooked by two Israeli settlements, Gilo and Har Gilo. Critics of the barrier say its intention is to take these settlements and as much of the surrounding land as possible onto the Israeli side in what is in effect a land grab.

In 2004, the International Court of Justice ruled that the route of Israel's barrier on Palestinian territory breached international law and was "tantamount to de facto annexation".

According to Israel's ministry of defence, "the route of the security fence in the Beit Jala region is based purely on security considerations. This portion … is there solely to keep terror out of Jerusalem." In a lengthy statement, the MoD recalled events of the second intifada, or Palestinian uprising, in which "Jerusalem was the prime target of a brutal Palestinian terror campaign; suicide bombings, shootings, and stabbings were weekly, and sometimes daily occurrences".

It said it was co-ordinating the route of the barrier with the local community, would provide an gate to allow access to land, and would pay compensation for land taken for the barrier's construction.

The ministry said the monks had initially requested to stay on the Israeli side of the barrier, and had only latterly demanded to be on the Palestinian side.

According to one source, the monks – who produce Palestine's only wine, under the Cremisan label – changed their stance following pressure from the Catholic hierarchy. Initially they believed being on the Israeli side of the barrier would give them continued access to the Israeli market for their wine, the source said.

Representatives of the monastery, whose presence at Cremisan dates back to 1891, declined to speak to the Guardian. However, after reports surfaced of tensions between the nuns and the monks over the route of the barrier, they issued a joint statement earlier this year "clearly affirm[ing] there is no discord among them whatsoever and that their positions with regard to building the 'wall' do not differ". The two religious communities, part of the Salesian order, "enjoy excellent relations and mutual respect", it added.

The half-dozen elderly nuns at the convent have objected to the barrier's route since first being notified six years ago. Under the present proposal, the convent's premises will abut the barrier. The playground of a kindergarten and school, run by the sisters for more than 50 years and catering for almost 400 Christian and Muslim children, will be overlooked by military watchtowers, and 75% of land owned by the convent will be on the other side of the barrier.

Manal Hazzan-Abu Sinni, a lawyer representing the nuns, said they were accustomed to a secluded lifestyle, and unfamiliar with legal procedures. "They feel there has been a great deal of intrusion into their day-to-day lives. From a remote, serene, spiritual place, they've become the focus of news and had to deal with a great feeling of uncertainty regarding the future," she said. "They're not used to dealing with such issues."

Yigal Palmor, spokesman for Israel's foreign ministry, said the involvement of a foreign government in a legal battle against another government was "very odd".