Government reveals bill for papal visit is up to £4m more than estimated, before policing costs are taken into account

Costs for the pope's inaugural visit to Britain are soaring, with taxpayers contributing as much as £12m towards the four-day event, an increase of £4m on previous estimates.

Lord Patten of Barnes, David Cameron's special representative for the papal visit, said at a Foreign Office briefing today that the government had revised its initial contribution because of a failure to grasp the "complexity and sophistication" of the trip.

He told journalists that non-policing costs were first put at £15m, with taxpayers shouldering £8m of the bill and the Catholic church paying the balance.

"These estimates were made in good faith. But I think we underestimated the complexity and sophistication of a visit that has normal aspects of a state visit and pastoral events. We will have to make a larger commitment even though we have driven down some of the costs."

Earlier this year the Cabinet Office told the Guardian that while there was no detailed breakdown of the figures, major costs were the hosting of state events in privately owned venues, scanning and bag checking inside venues, media facilities, and bed and board for the papal entourage, which is fairly modest and comprises top aides and a group of plain-clothes Swiss guards. On arrival this group will be joined by local senior clergy.

Patten added that the church would also have to raise more money for Catholics to get the "best possible experience".

Previously, the church had been due to contribute £7m towards the pastoral elements of the tour in September, such as the beatification of Cardinal Henry Newman, a prayer vigil in Hyde Park and an open-air mass in Bellahouston Park, Glasgow.

Archbishop Vincent Nichols, who was also at the briefing, said about £5m had been raised – around £1.1m from parish collections and almost £4m from private donors and other sources. He insisted the church would pay its fair share.

"I would like to stress from our point of view that not a penny is expected from public funds for those aspects of the visit which are an expression simply of the Catholic faith."

With three months to go before Benedict XVI arrives in Britain, the church is £2m short of its fundraising target and will have to find more if, as expected, the pastoral aspects cost more than expected. How much more remains unclear.

After the briefing Nichols said the church was prepared to revise its estimates as the government had done. When asked if this estimate would increase by between £2m and £4m, he indicated it was a possibility.

His spokesman was at pains to deny that the church contribution would increase by these amounts, saying that the target was still £7m but that this was "not a ceiling" and more money would be found if necessary. He also denied that the government was boosting its contribution to the visit to alleviate the financial burden on the church.

The papal visit of John Paul II, in the summer of 1982, was purely pastoral and left the church £6m in debt.

Policing and security costs remain a mystery. Lord Patten said they depended on the public turnout, which was "difficult to calculate". It is known that three forces – the Metropolitan police, Strathclyde and West Midlands – will be among those responsible for ensuring papal and public safety. The Met will draw on the expertise of its diplomatic protection group. All three forces must meet the costs from existing budgets.

The National Secular Society, is critical of state funding for the visit, said it would be using the Freedom of Information Act to find out how much was being spent on security in Birmingham, Glasgow and London. It claimed that the costs could "easily reach" £100m once security was taken into account.

It has been reported that the Met has already asked the Premier League to avoid scheduling high-profile matches during the weekend of the papal tour.

The police categorise games from A to C, with C being those most likely to result in trouble. Officers fear that any category C games during the visit could leave them without enough resources to police both events.

Commander Bob Broadhurst, of the Met, said: "We will ask that we do not have any problematic fixtures that weekend. Whether the people who make the fixtures agree to our request is a matter for them."

The papal visit has also affected the Tour of Britain, a cycle race across the country, with organisers moving the London leg from Westminster to Newham.

Nichols said there would be "careful consideration" of whether it would be appropriate for the pope to meet privately with victims of clerical sexual abuse. Any such meeting must not take place to "satisfy a public agenda or curiosity", he warned.