European stocks were mixed Wednesday although fading equity sentiment following a rare credit downgrade for China and the developing security situation in the U.K., where Prime Minister Theresa May increased the terrorist threat level to "critical" last night and warned that a new attack could be imminent, kept investors in a caution mood.

The region-wide Stoxx Europe 600 index held onto a modest 0.15% gain in the opening hour of trading, but regional benchmarks, including Germany's DAX performance index and the FTSE 100, drifted modestly lower. France's CAC-40 was marked 0.1% higher, thanks in part to solid gains for the country's biggest banks -- BNP Paribas SA (BNPQY) and Societe Generale SA (SCGLY)

The pound was marked little change from Tuesday's close at 1.2975 as trading began in Europe, with investors yet to factor-in changes to the country's terror threat level, which could mean as many as 5,000 troops deployed at public events over the coming weeks, after Prime Minister Theresa May increased the terrorist threat level to "critical" last night and warned that a new attack could happen at any time.

In equity markets, an early mover of note were shares in Fiat Chrysler (FCAU) - Get Report  , which extended declines after the U.S. Department of Justice said it is suing the Italian-American automaker over illegal emissions' management technology found in its vehicles.

The DoJ has filed a civil complaint alleging that Fiat failed to disclose engine management software found in its vehicles, at the certification stage, and that the engine feature in question does constitute a so called defeat device. Defeat devices are designed to mislead regulators over the true emissions' output of an engine when vehicles are tested in lab conditions.

Fiat stocks slid more than 3% during at the start of trading to change hands at an intraday low of €9.15, reducing its year to date gain to a little over 7%, before paring that decline to around 1.6% and a price of €9.34. The Stoxx Europe 600 Auto & Parts index was quoted just more than 1% lower shortly after the opening bell.

Marks & Spencer Group plc (MAKSY)  shares were also active, rising 0.6% higher to 390 pence each in the opening hour of London trading after the retailer posted stronger-than-expected full year profits and maintained its dividend.

M&S, as it is known in the U.K., said full year pre-tax profits came in at £613 million ($795 million), firmly ahead of the FactSet consensus of £592 million but down 11% from the previous fiscal year. Full year sales for the year ending in March were tabbed at £10.6 billion, a figure that was around 0.6% higher than last year and largely in-line with analysts' estimates. The company will maintain its annual dividend at 18.7 pence per share.

Elsewhere, the U.S. dollar notched a notable mini-rally overnight, with the dollar index rising 0.1% to 97.43, as investors reacted to the release of President Donald Trump's budget plans. The release of minutes from the Fed's two-day policy meeting earlier this month in which Janet Yellen and colleagues made no changes to the central bank's interest rate projections but indicated caution with respect to the slowing U.S. economy will dictate the dollar's path for most of the session.

Overnight in Asia, regional shares fell and the Australian dollar slumped after Moody's Investors Service lowered its sovereign credit rating on China to A1 -- the first reduction since 1989 -- amid concern for rising debt levels and a slowing economy.

The MSCI Asia ex-Japan index was marked 0.07% lower by 09:00 BST while the Nikkei 225 ended the session with a 0.7% gain to close at 19,742.98 points.

Global oil prices were firmer overnight, although not spectacularly so, with investors cementing bets that OPEC minister meeting in Vienna Thursday will announce an agreement to extend production cuts throughout 2017 and into the first quarter of next year.

WTI futures for July delivery were marked 18 cents per barrel higher, or 0.35%, at $51.65 in early European trading while Brent contracts for the same month were seen 0.5% from their Tuesday close in New York at $54.43 per barrel.

Wall Street looks set for a modest advance at the start of trading later today, according to early prices from U.S. equity futures, with the Dow Jones Industrial Average likely to gain around 18 points at the opening bell, or 0.08%, with similar percentage declines anticipated for the S&P 500 and the Nasdaq.