The Competition Appeal Tribunal is considering whether the Government was right to bypass competition concerns on the takeover of Halifax Bank of Scotland (HBOS) by Lloyds TSB.
The case is being brought by an organisation calling itself the Merger Action Group (MAG).
HBOS said it was confident the deal would be approved. HBOS shareholders will vote on the £12.2bn Lloyds rescue deal on December 12 and the approval from the Competition Appeal is expected to be made before then.
MAG is a group of businessmen, bank customers and shareholders who feel that allowing Lloyds TSB to buy HBOS will be bad for Scotland's economy.
It says that business secretary Lord Mandelson's decision to allow the deal to go ahead was based on new legislation that had not been in place at the time it was announced.