Members of the army join police officers outside the Palace of Westminster, London, after Scotland Yard announced armed troops will guard ‘key locations’ (Picture: PA)

The UK terror threat level is remaining at critical and hospitals across the country have been warned to prepare their staff for a potential second attack.

While media have not been briefed on a threat in a specific area, Theresa May has warned that a follow-up to the Manchester atrocity could be imminent.

thumbnail for post ID 6662716 NHS trauma centres told to prepare for possible second terror attack

Although there is no cause for undue concern about a second attack and police are making significant progress with the terror investigation – now seems like a good time you remind people that it is possible to alert emergency services to your whereabouts even in situations where you are unable to talk out loud.

EmergencySMS is similar to the police’s Silent Solution – which allows callers to inform emergency call handlers they require police assistance without making a sound.

But, the EmergencySMS service allows people to silently request urgent assistance from police, as well as paramedics, the fire brigade and the coastguard.

An armed police officer stands guard outside Buckingham Palace on May 23, 2017 in London, England (Picture: Getty)

Here’s how to do it:

Firstly, you must register your mobile phone number with emergency services before you can request help via text.

1. Send the word ‘register’ in an SMS message to 999
2. You will then receive SMS messages about the service
3. When you have read these SMS messages reply by sending ‘yes’ in an SMS message to 999
4. You will receive a SMS message telling you that your mobile phone is registered or if there is a problem with your registration.

Just to really push this point home – your phone number MUST be registered before you can use this service.

An ambulance arrives as police officers stand at the Miller Street and Corporation Street Crossroads, near the Manchester Arena on May 23, 2017 (Picture: Getty)

Be aware that the text service may take longer than a normal 999 call and it should only be used as a last resort – for example, if calling 999 and talking out loud would put you in further danger.

Once registered, hopefully you will never find yourself in a situation in which you require emergency services – but just in case below are a few simple steps to alerting them to your problem and location silently.

The SMS to 999 must include which emergency service you need, a brief description of the emergency and your location.

People stand near flower tributes for the victims of Monday’s bombing at St Ann’s Square in central Manchester, Britain, Friday, May 26 2017 (Picture: AP)

Emergency SMS provide the following example: ‘Ambulance. man having a heart attack. outside post office. Valley road Watford.’

Once you’ve sent your text you will receive a response which will either ask for further detail, or tell you that help is on the way.

thumbnail for post ID 6661368 Terror threat to remain at ‘critical’ with public urged to be vigilant

It is important to include as much information as possible while being as clear and concise to keep the back and forth to a minimum.

EmergencySMS advise not to assume your message has been received if you do not receive a reply.

It can take up to two minutes to get a reply to your initial message.

If you have not received a reply within three minutes then it’s best to send another text straight away.