This Sunday's London Marathon will proceed as scheduled - but with increased scrutiny of the crowds of spectators, following Monday's bombings at the Boston race.
The head of MI5, Jonathan Evans, met the Home Secretary Theresa May and the Scotland Yard assistant commissioner for specialist operations Cressida Dick to review security arrangements for this weekend's event, which is expected to draw half a million spectators onto the streets, lining the 26 miles from Blackheath to central London.
Both the funeral for Margaret Thatcher and Sunday's marathon are considered potential targets for terrorists, but intelligence officials have urged caution about reaching conclusions over whom exactly was responsible for the attacks in America killing two people and injuring 176 others with far-right groups as well as Islamists among the suspects.
The Boston Marathon route was swept twice for bombs, but that could not stop the explosive device being brought in. Random searches are due to be carried out in London in an effort to stop this.
Despite what happened at Boston, terrorist groups normally prefer to carry out their bombings in confined spaces in order to inflict large numbers of casualties. Vehicles would be kept away from the route of the London Marathon making it difficult to put a sizeable explosive device in place.
There also remains the threat of suicide bombers, but police cordons will be set up and the belief remains that those prepared to blow themselves up would want to maximise the numbers they kill and maim, not something suited to an open air location.
Furthermore, there is normally 'chatter' among terror groups and would-be terrorists, home and abroad, if a particular public event is targeted, about three to four months before they take place. The level of threat is ascertained from the veracity of the groups. There has not, it is believed, been any such discussion in suspect circles which has been detected about the London Marathon.
Such minor “precautionary adjustments” are being made to security plans, but no major changes will be made to what is already in place at a time when the terrorist threat to the UK is classified as “substantial”.
The security system put in place for the London Olympics proved to be effective and has been further refined since then. The law agencies hold that they would be adequate for both the coming events. However, some of the resources focused towards public order with protests due against the former prime minister may now be switched towards anti-terrorist measures.
Intelligence officials acknowledge that there is always the possibility that despite all the precautions taken and the application of tried and tested security principles, there may be an attempted attack.
“There could be an individual, or a group of individuals, who may try something somewhat impromptu; the hope, the expectation, is that they will be detected by what we have,” said one intelligence official.
“It may be a more sophisticated organisation, but these people know there will be quite stringent security, the alert after Boston, and the inherent risk they take in trying something in these circumstances.”
London Marathon's chief executive Nick Bitel said: “The support we have been offered by our stakeholders and the wider running community has been outstanding. We have the full support of the Metropolitan Police, the mayor's office and other authorities. We want to reassure our runners, spectators, volunteers and everyone connected with the event, that we are doing everything to ensure their safety and that the Virgin London Marathon 2013 is an outstanding success.”
Sports minister Hugh Robertson stated “ These are balance of judgments but we are absolutely confident here that we can keep the event safe and secure.I think this is one of those incidents where the best way to show solidarity with Boston is to continue and send a very clear message to those responsible.”