On both bridges a toll is applicable for all vehicles that enter Wales with prices charged at £5.40 for cars and vans, £10.90 for class two vehicles and £16.30 for class three vehicles (those with 18 or more seats).
"It's a tax on coming into Wales which has a negative impact on businesses based in Wales, those that want to invest or trade in Wales and the tourism industry," said Russell Lawson of the FSB.
The FSB want the Welsh Assembly to follow the lead of the Scottish government, which abolished all bridge charges at the beginning of the year.