LONDON (MarketWatch) -- BP's credit rating was slashed a staggering six notches by Fitch Ratings on Tuesday, just hours before its top U.S. executive is grilled in front of Congress as worries over how much the company will have to pay for the Gulf of Mexico oil spill grow.

BP's BP, -1.18% (BP) credit rating was knocked to BBB from AA, leaving the oil giant's rating at one of the three major agencies just two notches above junk status. BP was put on negative watch for further downgrades.

Fitch, which had cut BP's rating just two weeks ago, said the main driver was the Obama administration's insistence that claims being paid out of an escrow account.

"The recent claims by U.S. state and federal authorities that BP escrow significant sums pre-emptively, ahead of any agreed claims process, represent a material change in approach, should it ultimately prove a legally supportable move against the company," the rating agency said.

Fitch also noted that government scientists have increased their estimates of the spill rate.

Using a variety of company and industry estimates, Fitch sees immediate clean-up and claim settlements between $3 billion and $6 billion; civil fines of between $2 billion and $8 billion; "medium-term incremental one-off costs," which it didn't calculate but pointed out that a 1% rise in North American operational costs would increase spending by $700 million a year; and long-date litigation-related damages.

Fitch said it would be "surprised" if BP did not suspend quarterly cash dividend payments until the operational and financial impact of the incident is clearer.

The rating agency made clear it didn't expect BP to file for bankruptcy.

However, "the possibility of further, potentially multi-notch rating actions" arise from higher financial claims, the acceleration of the payment of claims and the possibility of declining BP's access to capital.

The rating agency action comes just hours ahead of a hearing by the House of Representative's Committee on Energy and Environment, which will feature BP's top U.S. executive, Lamar McKay, as well as executives from rival oil firms.

President Obama will make his first speech to the nation from the Oval Office at 8 p.m. Eastern.

In premarket trade, BP shares rose over 3%.

However, BP shares have lost nearly half their value since the Deepwater Horizon rig exploded, causing the Gulf of Mexico spill, on April 20.