Bosses at insurance giant Aviva will find out on Tuesday exactly how many customers have been affected by a blunder which saw confidential information about thousands of customers' finances sent to the wrong people.

Aviva, previously known as Norwich Union, has already asked some worried policyholders to destroy or return the misdirected letters, which reveal sensitive data such as policy numbers, dates of birth, and the annual bonus paid out on each policy.

But a spokesman for Britain's biggest insurance company has told the EDP it was still 'unclear' how many customers had been affected and said it was a matter being taken 'very seriously'.

He later added: 'I wouldn't like to speculate on the size of the problem because we won't know how many people are affected until Tuesday.'

The company has sent letters of apology to some of those known to have been affected and these arrived on Saturday.

The letter reads: 'We have recently issued annual bonus statements for the type of pension policy you have with us.

'Unfortunately, we have discovered that during the mailing process the address details used in the letter became misaligned from the information used in the rest of the letter.

'As a result you will have received a statement at your address which doesn't correspond to you or your policy. Please accept our apologies for this error.

'We are currently resolving the problem and will send you a revised statement within the next 14 days. You should keep this letter and the revised statement for your records.

'Please confidentially destroy the previous statement we sent you, or return it unopened to the address on the back of the envelope.

'We are sorry for this mistake and any inconvenience caused.'

The letter, which is dated April 8 and was sent from the company's Norwich headquarters, continued: 'Please be assured that Aviva takes all errors of this nature seriously, and have put in place actions necessary to prevent it from happening again and to ensure the safeguarding of your policy information.'

Worried customers seeking more information about the blunder are given the number of a helpline, although the letter adds that there will be no one taking calls over the Easter holiday.

Aviva provides 43 million customers with insurance, savings and investment products and is one of Europe's leading providers of life and general insurance. In 2011 its operating profit came in at �2.5 billion