A crisis gripping Goldman Sachs deepened today as Britain and Germany moved towards joining the US in pursuing a fraud investigation against the Wall Street bank for allegedly fiddling clients out of $1bn ($650m) through a misleading mortgage investment deal.

Gordon Brown ordered a special investigation into Goldman, accusing the bank of "moral bankruptcy". He threatened to block multimillion-pound bonus payouts if the firm is found guilty of wrongdoing.

In Berlin, Angela Merkel's government said it had sought information from the US Securities and Exchange Commission with a view to evaluating "legal steps" against Goldman.

Goldman's shares dived by 13% on Friday when the SEC charged the firm with collaborating with a hedge fund, Paulson & Co, to sell a deliberately skewed package of doomed mortgages in 2007, leaving clients including Royal Bank of Scotland and a German bank, IKB, nursing losses of more than $1bn. Paulson & Co made a fortune out of the deal by taking a short position to bet on the package's demise.

A London-based Goldman Sachs director, Fabrice Tourre, who is accused of masterminding the fraud, is still working as usual, although efforts by Sunday newspapers to track the Frenchman down to his flat near Sadler's Wells theatre in north London were unsuccessful. A Goldman spokeswoman said: "He's still an employee. He hasn't been suspended."

The case against Goldman has sent a ripple through the financial services industry, with analysts predicting it could be the first of many against similarly structured mortgage instruments known as collateralised debt obligations. The SEC's action took place amid wrangling in Congress over an overhaul of Wall Street regulation, where Republicans object to the scope of moves proposed by the administration.

Goldman could face fresh opprobrium on Tuesday, when it is due to publish its financial results for the first quarter of the year and is forecast to reveal revenue of more than $10bn – of which nearly $5bn could be earmarked for employee pay.

Brown told the BBC's Andrew Marr show that the case against Goldman fuelled his argument for a global tax on financial transactions: "I am shocked at this moral bankruptcy. This is probably one of the worst cases that we have seen.

"It makes me absolutely determined we are going to have a new global constitution for the banking system which I am pressing for, a global financial levy for the banks that all countries that are major financial centres pay, and we quash remuneration packages such as Goldman Sachs'."

Goldman vehemently denies any wrongdoing. The bank argues that it lost money itself on the mortgage deal, and that the clients involved were savvy investors. A source close to Goldman dismissed Brown's comments as electioneering: "It's no surprise, given that we're in the middle of an election campaign, that being tough on banks is popular."

The Liberal Democrat Treasury spokesman, Matthew Oakeshott, called for Tourre to be suspended following the "earth-shattering" allegations. The Tories also called for a crackdown.

Goldman's mortgage package was insured against default by Royal Bank of Scotland. Within nine months, 99% of the home loans involved had been downgraded, leaving RBS with an $841m bill.