Dissident republicans have developed the capability to mount an attack on the British mainland, according to the latest security assessment. Senior counter-terrorism sources confirmed the threat from dissidents attacking the mainland "now goes beyond an aspiration" and that they now possess the means to mount an attack across the Irish Sea.

Amid rising tension in the province and fears of an "Easter offensive" by dissident groups, police in Northern Ireland also warned that anti-ceasefire republicans were plotting to kill more police officers.

The increased threat from republican dissidents is certain to heighten security concerns during the build-up to the royal wedding on Friday, although there is no intelligence suggesting a specific plot related to the event.

On Friday another dissident grouping, styling itself "the IRA", issued a public statement claiming responsibility for the murder of PC Ronan Kerr in Omagh this month. The group, comprising former members of the Provisional IRA, vowed to embark on a bombing campaign. It is understood that the new group includes veteran paramilitaries who were involved in transporting and later detonating the bomb that exploded at London's Canary Wharf in 1996.

Intelligence officials monitoring dissident activity point to a growing sophistication in bomb-making techniques and a widening range of attack techniques as evidence of expanding capability. A senior intelligence source told the Observer: "We feel there is capability to attempt some form of an attack on Britain. Based on our assessment, it goes beyond an aspiration." Dissident groups have recently deployed command-wire explosive devices, van-mounted weaponry, car bombs and vehicle booby traps, as well as more orthodox military equipment such as hand-grenades. Several individuals are believed to be under surveillance.

The mainland has not experienced an Irish republican attack since car bombs exploded at the BBC Television Centre and Ealing Broadway station in London in 2001. The head of MI5, Jonathan Evans, said last September, however, that dissidents posed a "real and increasing security challenge in Northern Ireland" and could be planning attacks elsewhere. According to MI5's Joint Terrorism Analysis Centre, the official threat level is "substantial", meaning an attack is a "strong possibility".

On Saturday a man appeared in court facing charges in connection with the murder of Kerr. Gavin Coyle, 33, from Omagh was charged with possession of explosives, firearms and articles likely to be of use to terrorists. He was remanded in custody. The court heard he was linked to a footprint found at a major dissident republican arms dump in Coalisland during investigations into Kerr's murder. Police have also revealed details of a substantial haul of guns and ammunition found in a vehicle stopped by officers in Keady, near the Irish border, on Friday.

Attention has concentrated upon the Real IRA and the smaller but technically able Oglaigh na hEireann, which has improved its explosives technology over the past two years. Analysis suggests that the explosives material being used by dissidents may have originated from a onetime Provisional IRA stockpile whose whereabouts were known by former quartermaster general Michael McKevitt – who formed the Real IRA.

Police in Northern Ireland said yesterday that fresh violence was expected. "Dissident terrorist groups are continuing to identify officers and target them with the single objective of killing them," a spokesman said.

In further evidence of growing confidence among extremist republican groups, a leading figure in one of the dissident groups' political wings announced that the Queen should be considered a "legitimate target" during her visit to Ireland in May. The general secretary of the hardline Republican Sinn Féin party, Josephine Hayden, said she would have no problem with a sniper targeting the Queen. "You might say that she is just a little old grandmother," said Hayden, "but it is what she represents, what she symbolises that counts. She is a legitimate target."

The Observer has learned that a radical republican group known as Eirígí: for a Socialist Republic is planning to occupy Dublin's Garden of Remembrance 48 hours before the Queen is scheduled to attend a reconciliation ceremony there.

Republicans in Dublin say the splinter group plans to establish a tented camp on the Sunday prior to the visit, creating the possibility that the Garda Siochána will have to forcibly remove protesters before the royal tour begins on 17 May.

On 5 May, Northern Ireland is braced for trouble to mark assembly elections and the 30th anniversary of the death of IRA hunger striker Bobby Sands. Future trouble could depend on the reaction from the loyalist community, described by sources as "relatively restrained" until now. A 40-year-old Belfast man was, however, arrested on Saturday in connection with loyalist terrorist activity.