British citizens and other eligible individuals have been told their UK passport applications could face delays or even rejection. As we approach a bustling summer travel season, His Majesty's (HM) Passport Office has issued a warning, urging those applying for either a first-time or replacement passport to stick to specific guidelines.

A message from the official HM Passport Office X account on Monday reads: "Your passport is your most important travel document - it's worth taking your time to get right! Before submitting your application check your photo meets all requirements; all personal details are correct; supporting documents are ready; payment details are accurate."

UK passports can be applied for by British citizens, British subjects, British nationals, British protected persons, or British overseas citizens or overseas territories citizens. Applications may face rejection or delay if your photograph fails to comply with certain regulations.

As per Government advice, you can use a digital photo, either from a photo booth or shop, or take one using your own device. Photos must be of high quality, clear and in focus, in colour, unedited, and of a suitable resolution size, but not exceeding 10MB.

The photo should not include any other visible people or objects, and you should stand out against the background - which ought to be plain and light-coloured. You should also ensure there is no "red eye" in the image.

In your photograph, you should be facing forward, staring directly into the camera with a closed mouth and neutral expression. Your hair must not obstruct your eyes, which should be open and fully visible.

Head coverings are not permitted unless for medical or religious reasons. There should be no shadows on your face or behind you, and glasses should not be worn unless necessary.

If glasses are required, they must not be sunglasses or tinted, and your eyes must not be obscured by the frames or any shadow, glare, or reflection. The guidelines differ slightly for young children, although children must be alone in the photos, with babies not holding toys or using dummies.

Children under six do not need to be looking directly at the camera or have a neutral expression. Babies under one do not need to have their eyes open.

You can support their head with your hand, but your hand must not be visible in the photo. Babies under one should lie on a plain light-coloured sheet, with the photo taken from above.

Further advice on Gov.uk adds: "You must get a new photo when you get a new passport, even if your appearance has not changed. Your photo must have been taken in the last month. Your application will be delayed if your photos do not meet the rules."