TalkTalk has received a ransom demand from someone claiming to be behind a cyber attack in which millions of customers' bank details are feared to have been stolen.

The company, one of Britain's biggest communications firms, announced last night that it had suffered a 'significant and sustained cyber attack' in which the bank, credit card and personal details of all of its four million customers may have been accessed.

The company said this morning that it has now been sent a ransom demand by suspected hackers.

A spokesman for the firm said: 'We can confirm we were contacted by someone claiming to be responsible and seeking payment.'

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Baroness Dido Harding, head of TalkTalk, which has suffered a 'significant and sustained cyber attack'

The company has refused to say how much money is being demanded or whether they know who the hackers are.

TalkTalk's chief executive, Baroness Dido Harding told the BBC: 'Yes, we have been contacted by - I don't know whether it's an individual or a group purporting to be the hacker.

'I personally received a contact from someone purporting - as I say, I don't know whether they are or are not - to be the hacker, looking for money.'

Baroness Harding previously said the company had assumed a worst case scenario that all the personal data relating to its four million customers was compromised until they could confirm exactly what was taken.

It came after an expert on computer crime revealed that an extremist group based in the former Soviet Union had claimed responsibility.

Adrian Culley, a former detective in the Met's cyber crime unit, told BBC Radio 4's Today Programme: 'They are claiming to be from Soviet Russia and be an Islamic cyber jihadi group. They have posted [online] information that appears to be TalkTalk customer private information.'

His comments came after a garbled message was put online which purported to be from those behind the hack.

It stated: 'We Have adapted To The Security measures Of The Web,, We Cannot Be Stopped... We Will Teach our Children To Use The Web For Allah.. Your Hands Will Be Covered In Blood.. Judgement Day Is Soon.

'WE Are In The Soviet Russia And Near Place, Your Europe, WE control Asia, We Control AMERICA.'

The accuracy of the information has, however, not been verified.

Message: The phone and broadband provider has asked all customers to change their passwords and check for any unusual transactions from their bank accounts

Professor Peter Sommer, from De Montfort University's cyber security unit, said: 'It seems to me the suggestion that these are Islamic terrorists who are perpetrating it is unlikely, not impossible.

'One has to look at what is probably the most likely outcome. One of them is an extortion attempt; since they have gone public I suspect that's not going to work. The other one is just to get hold of the credit card information, get hold of the personal information.'

ATTACK IS THE LATEST IN A SERIES OF SIMILAR ISSUES FOR THE COMPANY IN RECENT MONTHS This is the third in a spate of cyber attacks affecting TalkTalk customers. In August the company revealed its mobile sales site was hit by a 'sophisticated and co-ordinated cyber attack' in which personal data was breached by criminals. And in February TalkTalk customers were warned about scammers who managed to steal thousands of account numbers and names from the company's computers. The theft of data was discovered when TalkTalk investigated a surge in complaints from customers about scam calls between October and December 2014. The company has said the three previous attacks on the firm were 'completely unrelated'. Advertisement

After TalkTalk confirmed the attack last night, the company asked all customers to change their passwords and check for any unusual transactions from their bank accounts.

Cyber security experts are warning customers to be wary of anyone who emails or phones them purporting to be from the company and asking for bank details or passwords.

Jon French, security analyst of AppRiver, said: 'The two major things customers need to do is keep an eye on their banking information to look for fraudulent transactions, as well as be vigilant with communications.

'By communications, I mean they should be suspicious of any unexpected emails or phone calls that may be asking them for additional information.'

David Richards, founder and CEO of software firm WANdisco, told MailOnline there is a blackmarket for such information, which could be sold on to fraudsters.

He said the information was often unwittingly sold on to marketing firms, meaning TalkTalk customers could also face being deluged with spam emails.

Mr Richards hit out at firms who do not strongly encript their customers' data, adding: 'It's like someone breaking into your house and then finding the keys in the safe.'

Jason du Preez, CEO of data privacy company Privitar, called the hack a 'high-profile reminder' that it is impossible for companies to protect data with traditional security methods.

Mr du Preez said: 'These hacks are not just embarrassing to the organisations involved. They can have really serious financial and personal consequences for your users, destroying consumer trust and loyalty.'

This is the third in a spate of cyber attacks affecting TalkTalk customers - and Mr du Preez warned that hackers were always creating new ways to attack websites.

He said: 'Cyber security is a cat and mouse game. Every time better security systems are built, hackers step up their game and find new ways to beat them. Even the most secure systems can become vulnerable.'

Advert: TalkTalk said it had contacted major banks which will monitor any suspicious activity from customers' accounts and had informed the data protection watchdog, the Information Commissioner's Office

WHO IS THE WOMAN AT THE HELM OF TELECOMS GIANT TALKTALK? Diana Mary "Dido" Harding, 47, was born in Dorset and is the daughter of retired British Army Officer Lord Harding Dido Harding has been thrust into the spotlight with her company TalkTalk being targeted by a cyber attack - potentially leaking sensitive data belonging to millions of customers. But who is the woman at the helm of the telecoms giant? Diana Mary "Dido" Harding, 47, was born in Dorset and is the daughter of retired British Army Officer Lord Harding, Baron Harding of Petherton. Her grandfather John Harding, 1st Baron Harding of Petherton, led the Desert Rats in the Second World War. Mrs Harding was made a Conservative life peer last September, taking the title of Baroness Harding of Winscombe. She is married to Tory MP for Weston-super-Mare, John Penrose, and they have two daughters together. Mrs Harding is a racing enthusiast and her horse, Cool Dawn, won the 1998 Cheltenham Gold Cup. She graduated from Oxford University with a degree in Philosophy, Politics and Economics, before going on to gain an MBA from Harvard Business School. Mrs Harding became chief executive of TalkTalk Telecom Group in 2010, having previously been Sainsbury's convenience director. She joined the supermarket from Tesco where she held a variety of senior roles both in the UK and international businesses. Before her stint at Tesco she worked at Kingfisher and Thomas Cook. In July 2014, Mrs Harding was appointed as a non-executive director on the Court of The Bank of England. She has also served on the boards of The British Land Company PLC and Cheltenham Racecourse. Advertisement

Shares in the company fell by 4% during early trading this morning after the announcement late last night.

A TalkTalk spokesman said: 'We are continuing to work with leading cyber crime specialists and the Metropolitan Police to establish exactly what happened and the extent of any information accessed.'

The Metropolitan Police confirmed its Cyber Crime Unit was investigating an allegation of data theft which was reported yesterday, adding: 'There have been no arrests and inquiries are ongoing.'

There is a risk that all of our customers' personal data has been accessed and therefore we are taking that very seriously Dido Harding, TalkTalk chief executive

The TalkTalk website was unavailable to customers last night and displayed the message: 'Sorry we are currently facing technical issues, our engineers are working hard to fix it. We apologise for any inconvenience this may cause.'

The company spokesman added: 'A criminal investigation was launched by the Metropolitan Police Cyber Crime Unit following a significant and sustained cyber attack on our website yesterday.

'That investigation is ongoing, but unfortunately there is a chance that some of the following data has been compromised: names, addresses, date of birth, phone numbers, email addresses, TalkTalk account information, credit card details and/or bank details.

'We are continuing to work with leading cyber crime specialists and the Metropolitan Police to establish exactly what happened and the extent of any information accessed.'

Concerns: TalkTalk, which has more than four million customers in the UK, said credit card and bank details along with personal information may have been accessed during the attack

In a letter to customers, TalkTalk managing director Tristia Harrison said the company took 'any threat to the security of our customers' data very seriously'.

'Unfortunately cyber criminals are becoming increasingly sophisticated and attacks against companies which do business online are becoming more frequent,' she said.

TalkTalk said it had contacted major banks which will monitor any suspicious activity from customers' accounts and had informed the data protection watchdog, the Information Commissioner's Office.

It is also organising free credit monitoring for a year for all of its customers.

The company said any customers who notice unusual activity on their accounts should contact their bank and Action Fraud, the UK's national fraud and internet crime reporting centre.

They have also been urged to change their TalkTalk account passwords and any other accounts which use the same passwords.

TalkTalk said it is working with credit reporting service Noddle to offer 12 months of credit monitoring alerts for free.

A TalkTalk spokesman said: 'We are also working hard to get our services back up securely so they are available for customers to use safely and securely as soon as possible.

'We are extremely sorry for any concern and inconvenience this incident may have caused.'