As Apple Pay is rolled out successfully in U.K., it is welcomed as a boon by several people in the country for making petite payments such as for tube travel, pound stores, confectionaries, and coffee shops

However, the Transport for London has issued an advisory for all Apple Pay users who wish to use the service while on the go, to save them some embarrassment and money.

The advisory as stated in a news article published on Mobile Choice UK this Wednesday asks the commuters to ensure that their "Apple Watch and iPhone are adequately charged to complete the entire journey".

If in case the gadget's battery drains out in the middle of a rail or tube journey, users won't be able to touch out and thus, could end up paying the maximum fare. Likewise, ticket inspector also cannot read their device, making the customers accountable for penalty fare.

"Where it is possible, we will try our best to help the customer complete their journey automatically in a similar fashion we do with Oyster and contactless.

"In case, the journey cannot be completed automatically, customers can opt for refund. For this, they ought to log into their Apple Pay account online or may choose to contact the customer support team, but it is always recommended that the device be charged well before getting aboard," says Transport for London in the advisory.

The term "Touching out" indicates holding the Apple Watch or iPhone 6 against the Apple Pay reader for the system to figure out correct and total fare as well as to make payments. So watch out before getting engrossed with that battery gulping music app on the iPhone or checking out some apps on that smart Watch. Don't want to pay additional or maximum fare for a rail or tube journey? Make sure the battery bars are at decent and adequate levels to complete the entire journey.

According to an article posted on UK's daily website Cult of Mac, failing to comply with the Transport for London's advisory can be meted the maximum fare charge that can go up to £8.80 or more. In addition, things can turn very expensive if customers are confronted with a ticket inspector and the iPhone or Watch is dead. During such times, the penalty fare can go up to £80, terms and conditions apply.

Considered yourself warned.