With the majority of people planning to do at least part of their Christmas shopping online, the recent attack on 9,000 Tesco Bank accounts is a timely reminder that you can’t be too careful when carrying out cyber transactions.

Next weekend will be the most lucrative of the entire year for internet retailers. According to price comparison website Gocompare, UK households will spend £4.9 billion – an average of £175 each – between what are now known as Black Friday and Cyber Monday.

But cyber criminals will be out in force too, ready to exploit any weaknesses they can to steal card details and hack more bank accounts.

Jody Baker, head of money at comparison website CompareTheMarket, said: “Our research shows that one in ten people’s bank accounts had fallen foul of a cyber-attack over the last year – the equivalent of 4.5 million accounts – and had an average of £475 stolen.

“With the busy Christmas period soon upon us, we would expect to see a spike in the number of online frauds in the coming weeks.”

Tesco had to repay £2.5 million to customers after hackers breached its security earlier this month and began helping themselves to their money.

Matt Sanders, Gocompare’s head of credit cards, said: “The Tesco Bank hack is just the latest in a string of attacks from cyber criminals and follows not long after the worst-ever hack inflicted on a company, which saw the personal details of at least 500 million Yahoo! accounts stolen by hackers.

“While these breaches of security are not the fault of customers, they do act as a vital reminder about the importance of actively managing passwords and PINs to keep our information safe and secure – especially as we enter the Christmas shopping period.”

He added: “For a strong password, we recommend you use a minimum of six characters, which include a combination of upper and lower case letters, numbers and symbols, such as exclamation or question marks.

“And, remember to use a different PIN or password for each account or application you access. Otherwise, if your password is stolen, criminals will have access to all your online accounts.”

Consumers are advised not to use the same passwords for social media accounts and email, and to avoid predictable words and phrases like family names and birthdays.

It is also vital to ensure that software on all devices is up to date so that security upgrades are applied. Effective anti-virus protection should also be applied and firewall settings should be turned on.

Shoppers should also never make purchases on shared computers or public Wi-Fi, and should buy only from secure websites. The address, or URL, should begin with “https” rather than just “http” and there will be a closed padlock symbol in the address bar.

Emails advertising offers or discounts that are too good to be true should be treated with scepticism because there is a high chance the items will be counterfeit or the link will lead to a fake site.

Copycat sites created to fool consumers into parting with their financial details may be quite convincing at first glance, but subtle differences and spelling and grammatical mistakes tend to give them away.

Before making any purchase, shoppers should always read a website’s terms and conditions, delivery information and returns policy, and note the site’s contact details in case of problems later. Reputable businesses will provide a head office address, not just a PO box, email address or mobile number.

It is also never advisable to let a website store card details for future purchases. It may take a few extra seconds to input the details each time, but if could act as a good protection against fraud.

For purchases over £100, using a credit card provides additional protection. If goods do not arrive, are damaged or faulty, or the company goes bust, the card company will reimburse the amount spent.

Paying this way can also help with budgeting. Mr Sanders said: “A zero per cent credit card for purchases can help you to spread the cost for your shopping over a certain amount of time with no additional interest.

“Likewise, a zero per cent balance transfer card could help you cope with a financial hangover by letting you pay off your purchases over an extended period of time.”

However, he warned: “In both cases, you need to be careful to repay the debt before the introductory terms end or you’ll have to start paying interest. Set up a repayment plan to ensure you pay off the balance before this period ends, and remember to check your card’s terms and conditions.”

For smaller purchases PayPal could be a good option. It is free to use, setting up an account takes minutes and it will enable customers to shop without sharing card or bank details.

At this time of year, when shoppers are likely to be carrying out more transactions than normal, it is important to keep a close eye on bank accounts too.

Ms Baker said: “It is a good idea to regularly check your bank statements for any unusual activity, as criminals often make small but regular thefts which are harder to spot than larger one-off purchases.”