Thousands of British savers with billions of pounds in products they thought were safe were left exposed by last week’s crisis on Wall Street.

Three of Britain’s biggest banks have almost half a billion pounds of exposure to Lehman Brothers, the US investment bank that filed for bankruptcy last week. This could affect scores of UK investors with so-called “protected” products, according to analysis by data provider Mtn-i.

There are another 14,000 savers with almost £11 billion in bonds with American International Group (AIG), the insurer that was thrown an $85 billion (£47 billion) lifeline by the US government. The schemes, including “guaranteed” income bonds, were sold primarily to wealthy savers with at least £250,000 to invest.

The company barred some savers from withdrawing their holdings for up to 90 days last week, amid confusion over whether their cash was safe and how much protection they would have under the Financial Services Compensation Scheme (FSCS).

AIG insisted its British products were “ringfenced”, but questions nevertheless remain over whether the FSCS would give savers as much protection as they expect.

The FSCS conceded last week that savers might not have a claim on investment growth or “guaranteed” returns yet to be paid. In other words, if you invested in a guaranteed-income bond paying a fixed annual return each year which was rolled up until the end of the contract, only your original investment might be covered.

“If a product is due to pay out, but hasn’t done so, you could only claim for the initial financial loss,” a spokeswoman said.

Meanwhile, Lloyds TSB and HBOS (Halifax Bank of Scotland) — which agreed a £12.2 billion takeover last week — and Royal Bank of Scotland collectively have £450m of outstanding contracts with Lehman. Many of these underpin so-called structured or protected products in which savers get a portion of market growth in return for some or all of their capital back — unless the underlying firm goes bust. Many savers now face an anxious wait to find out what will happen to their cash.

Christopher Powell of Arc, which offered four structured products backed by Lehman, said he was unsure whether clients would receive their next income payment.

Fears about the compensation scheme were also raised by events at HBOS, whose shares went into freefall on Wednesday. Savers rushed to pull out cash above the £35,000 compensation limit, raising the spectre of a run on the bank and prompting the Lloyds merger.

Thousands of people have savings at both institutions and may now have just one £35,000 limit.

Also on Wednesday, thousands of investors in exchange-traded commodities (ETCs), a type of share, found their AIG-backed investments had been frozen. Trading is slowly returning to normal but, as with all shares, ETCs would not be protected by the FSCS in a worst-case scenario.