Shareholders in listed accountant RSM Tenon face being finally wiped out if rival Baker Tilly proceeds with its potential offer, experts are warning.

They say a merger could also threaten up to 150 jobs in Scotland, where the two firms employ 500 between them, and negatively impact dozens of sizeable Scottish plcs, which signed up for Tenon's closed tax avoidance schemes.

Lloyds Banking Group, however, will be hoping to recoup some losses. It owns 10% of the equity, currently valued at under £5.5 million, but has Tenon on life support with a £90m loan facility.

Baker Tilly has until August 22 to decide whether to make a cash offer but could decide as early as this week.

RSM Tenon, the only quoted accountant and the seventh largest, promotes itself as the UK's biggest provider of bankruptcy services. But it has been fighting its own meltdown since its shares crashed from 66p to 6p in the 12 months prior to the departure in January last year of chairman Bob Morton and chief executive Andy Raynor, and the unveiling in March 2012 of 300 redundancies, an £84m loss after a £61m writedown, and a £12m restatement of its own accounts.

A year ago the Accountancy and Actuarial Discipline Board (AADB) launched an investigation into the group's internal and external auditors over Tenon's £76m acquisition of rival RSM Bentley Jennison and admission to the main market in 2010.

Tenon has a turnover of around £200m, while Baker Tilly, seen as the only credible buyer of Tenon, is just behind Tenon in eighth position in the sector's top 10 with £170m.

In February Tenon said it had entered into discussions with Lloyds to avert the "significant risk of a facility breach" over the next 12 months, highlighting a 10% fall in revenues, In May it said discussions were continuing, though Lloyds remained supportive of the group as a going concern.

The announcement of Baker Tilly's unsolicited approach came on July 25, three weeks after Tenon's financial year end.

One industry source commented: "The bank probably decided to give Tenon until its year end to show that it was becoming profitable again and could start to pay down the debt. Perhaps it turned to the only show in town, Baker Tilly, also a customer, and asked it to get ready to look at the books."

On the potential outcomes he said: "The bank will not want a penny going to shareholders. By far the best option for Baker Tilly would be to buy Tenon's trade and assets. It gets a tax deduction against profits for the purchase of goodwill , it can cherry-pick what it wants to buy, and the unpleasantness of making people redundant would fall squarely on Tenon's shoulders. The bank would get all of the proceeds and Baker Tilly would be forced to take some of the debt, but that still represents a good result for it."

The Herald revealed earlier this year that a raft of Scottish corporate clients of RSM Tenon face combined tax bills of up to £400m in proposed settlements with HM Revenue & Customs over employee benefit trust (EBT) tax schemes.

Premier Strategies, a former division of Tenon, is estimated to have sheltered up to £1 billion for dozens of sizeable Scottish companies, avoiding £400m of taxes, in the decade to 2012 before Tenon closed Premier down, leaving the avoidance market to smaller players.

It followed chancellor George Osborne's promise to introduce the general anti-avoidance rule which has recently been enacted.

Premier is estimated to have done half of its business out of Tenon's three Scottish offices, particularly Glasgow and Aberdeen.

It hiked its sales from £2m to £37m over the six years to 2008, earning a £3.6m pay-out for Premier co-founder Mark Edmond that year: twice the aggregate remuneration of the RSM Tenon board. Mr Edmond, along with Scottish advisers Peak Performance, whose clients include comedian Jimmy Carr, have been pursuing RSM Tenon for unpaid bonus or commission, according to the group's last accounts.

In March RSM Tenon said its staff were still handling inquiries from former Premier corporate clients, despite having no dedicated staff in the area.

One tax adviser said he believed there were "several dozen" Scottish mid-sized firms, clients of Premier, facing potential "punitive tax or fines, which could threaten their businesses."

He added: "Premier is now a shell firm and would be left behind. If Tenon staff join Baker Tilly, they will not be able to spend non-chargeable time as they do at present helping ex-Premier clients deal with ongoing HMRC enquiries."

Among shareholders nursing losses are fund group Jupiter, which holds more than 8%, and the former chairman and activist investor Bob Morton, with almost 7%. RSM Tenon shares have halved since the talks were announced and closed on Friday at 1.75p.

Lloyds declined to comment.