The takeover of HBOS by Lloyds TSB will come under fresh pressure this week as the Halifax and Bank of Scotland owner faces a possible counterbid from a mystery foreign suitor while having to write off an estimated £5 billion because of falling asset values.

Jim Spowart, the former chief executive of HBOS’s Intelligent Finance division, confirmed yesterday that he had been approached by an “international financial institution” with a view to brokering a bid for the bank that would trump the Lloyds TSB offer.

News of the possible counterbid intensified pressure on Alistair Darling to clarify his attitude to a second offer. But Treasury sources claimed that it was not a matter for the Chancellor but for the HBOS board, which has already agreed the £12 billion merger with Lloyds TSB, and its shareholders.

The Government was forced to defend itself against accusations by a senior SNP member that it leaked the potential counterbid in order to scupper it.

Alex Neil, an MSP and member of Holyrood’s finance committee, noted that Mr Spowart held private talks with Jim Murphy, the Scottish Secretary, last week to seek assurances that all bids would be assessed on a “level playing field”. Mr Neil accused Mr Murphy of breaching the ministerial code by leaking the possibility of a counterbid so that the Government’s preferred takeover by Lloyds TSB would go ahead. A spokesman for the Scotland Office insisted that Mr Murphy had “at no point divulged any commercially sensitive information”.

Mr Spowart refused to identify the putative bidder, but confirmed that it was one of the parties he had spoken to in September when he looked at a possible rescue of Bank of Scotland. He added: “This time it wouldn’t be a hive-off of Bank of Scotland.”

He confirmed that he had told the mystery suitor that, in the event of a successful bid, he would be able to assemble a board including high-profile bankers. The mooted Bank of Scotland rescue was said to have involved Sir George Mathewson, former chief executive of Royal Bank of Scotland (RBS), and Sir Peter Burt, former chief executive of Bank of Scotland.

Mr Spowart admitted that the leaking of the bid was unhelpful, but insisted: “At this stage, it’s a serious intent.”

He may now face demands from the Takeover Panel to clarify whether a counterbid for HBOS is a real prospect. A spokesman for HBOS said: “We never comment on market speculation. We have a recommended deal with Lloyds TSB which brings certainty and tangible benefits to our shareholders.”

Analysts said that pressure on the deal with Lloyds TSB would be increased this week when HBOS reveals an estimated £5 billion writedown, highlighting its need for fresh capital.

Separately, RBS is expected to unveil a further writedown of at least £3 billion in the second half of the year, on top of the £5.9 billion written off in the first half.

Alistair Darling will be questioned today by the Commons Treasury Committee over the risks to the taxpayer from the Government’s banking bail-out. The Chancellor is expected today to unveil details of the arm’s-length agency that will manage the Treasury’s £37 billion stakes in Britain’s banks, and seek to ensure that they maintain lending to small businesses and homebuyers.

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