The security operation surrounding the Duke of Edinburgh’s funeral is likely to be on a “substantially smaller scale” than previous ceremonies, according to a former counter-terror chief.

Nick Aldworth said the security operation may be “probably the easiest that we are likely to ever see for a royal funeral” as the event has been pared back amid the coronavirus pandemic.

The former counter-terrorism national co-ordinator, who used to have responsibility for such events, told the PA news agency: “This will be a substantially smaller-scale operation than would have happened previously.

“My view is that this is a fairly simple security operation and probably one conducted within mostly existing resources.”

The nature of this funeral is that almost all of those risks will have been mitigated by the fact that it is effectively a small family funeral in an already secure area

The duke is to be honoured with a ceremonial royal funeral at St George’s Chapel in the grounds of Windsor Castle on Saturday.

Crowds are being urged to stay away from Windsor and only a small number of guests will attend the service, with even the Prime Minister stepping aside to make way for an additional family member under the tight rules on numbers.

As it is taking place in an area where the royal family often live and worship on a routine basis, there will be a “degree of business as usual around this event”, said Mr Aldworth, who also used to police the town.

Throughout the week, police have been carrying out searches with armed and horseback patrols in the town as preparations continued.

Thames Valley Police said this included searches of phone and post boxes, drains and bins as well as number plate checks, monitoring of CCTV and installing barriers to prevent attacks with vehicles, alongside covert tactics.

Although expecting the town to be busier than normal with people arriving to pay their respects, police have asked the public not to gather by royal residences, to avoid meeting in large groups, to minimise travel and to make a donation to charity instead of laying floral tributes in a bid to prevent the spread of Covid-19.

This is amid current lockdown rules banning groups of more than six people or two households outside in England.

Mourners are instead being pointed towards an online book of condolence to pay their respects.

Funerals of senior royals are years in the planning, with each operation named after a different bridge.

The Queen is thought to be London Bridge and the duke is Forth Bridge.

Mr Aldworth said the plans are of such a scale they could sometimes even exceed police resources.

They may typically involve a convoy, lying in state, hundreds of visiting dignitaries and vast numbers of guests and contributors, as well as mass crowds.

But a small, self-contained funeral inside an historic castle “dramatically reduces the security footprint”, he said.

The security operation will focus on preventing risks and “embarrassment”, particularly controlling unauthorised access to the castle.

Outside the grounds, Mr Aldworth expects a “relatively discreet and relatively low-scale policing presence”, mainly for enforcing Covid-19 laws.

But police and security services will be alert to any incidents that could prevent a threat – from accidents, public disorder and crime to terrorist attacks.

The potential for knife attacks, shootings and the ability to plant bombs will all be considered.

Mr Aldworth added: “The nature of this funeral is that almost all of those risks will have been mitigated by the fact that it is effectively a small family funeral in an already secure area.”

But officers may be particularly “alive” to threats drones could pose in disrupting or attacking the ceremony and can draw on technology to try to scupper their flight path or even call in the military.

Richard Aitch, director of operations for firm Mobius International, which offers security services for governments and private companies, told PA there could be a focus on covert tactics to avoid overshadowing the sombre mood of the day.

He believes that even in a pandemic, the security operation for an event like this would be “huge”, with the cost running “into the millions”.

It could also involve GCHQ listening in to “chatter” around the world online and on phones, looking out for any spikes in conversations about the funeral, while vetting would be a “huge aspect” of the arrangements, he added.

The widespread use of face masks amid the pandemic may present challenges to police as they make it harder to identify people, he warned.

Another operation will continue around Buckingham Palace and Westminster over the weekend.

Metropolitan Police officers will also be deployed to a demonstration planned in central London on Saturday afternoon.

Protest is permitted under current Covid-19 regulations but plans must be put in place to ensure the gathering is safe and minimises the risk of transmission, the force said in a statement.