Anti-virus company McAfee claims an attack could result in 2.2 million office days lost in downtime.
It says that the worst hit will be small and medium sized businesses which had failed to have adequate virus protection.
Despite media publicity about viruses in recent years, keeping AV protection software up-to-date is something that is widely ignored by SMEs, the report said.
The £2.1bn figure was based on the impact of previous virus attacks on SMEs, McAfee said.
The company estimated that 70% of all SMEs had received a virus, with each losing an average £843, and 7.2 hours in lost computer time.
Only half of all respondents regularly schedule virus scanning of company computers, it added.
Barclays has partnered with accounting software company Xero to provide businesses with access to transaction data through its direct feed.
Government's estimate of a £400m admin saving from Making Tax Digital is way off - and is instead a huge cost burden, warns Lamont Pridmore chief executive Graham Lamont
Xero unveiled its expanded global partner programme at Xerocon South, the accounting technology conference in Australasia
Accountancy software firm Sage has been hit by a data breach which may have compromised the personal details and bank account details of as many as 300 UK businesses