UEFA is to keep Ukraine and Poland as co-hosts of Euro 2012 but warned both countries Friday that "any slackening" of efforts could still put the tournament in doubt.

The number of venues for the tournament could also be reduced from eight to six, the executive committee of football's European governing body decided in Bordeaux, France.

The event could go even go ahead with one country having more venues than the other, rather than with an exact sharing of host cities.

UEFA president Michel Platini said: "It would, of course, been easier to go to Germany or Spain, but it is 99 per cent certain. Patience is needed but we are full of anticipation."

UEFA has made the completion of the Kiev and Warsaw stadiums "an essential element of the whole project," a statement said.

"If we can't play in Kiev and Warsaw there won't be a European Championships in Poland and Ukraine," Platini said.

The governments of both countries are being urged to cooperate fully with the national football associations on improving infrastructure amid concerns over transport, poor airport facilities and a shortage of hotel accommodation.

In May next year, the executive committee will decide whether the co-hosts have fulfilled criteria relating to Poland's venues in Warsaw, Gdansk, Poznan, Wroclaw, and Ukraine's in Kiev, Lviv, Dnepropetrovsk and Donetsk.

A UEFA statement said: "Both host countries must continue to make the necessary efforts as any slackening could put in doubt the organisation of this tournament in these countries."

Former German international Franz Beckenbauer, attending the meeting as a FIFA executive committee member, ruled out any prospect of Germany stepping in with alternative venues.

Despite speculation over Berlin or Leipzig as possible venues, this was "not an issue," he said.

The UEFA executive meanwhile agreed to expand the European Championships to 24 teams from the current 16 from 2016.

The format of the final tournament will consist of six groups of four teams, followed by a round of 16, quarter-finals, semi-finals and the final.

The top two from each group would qualify in addition to the four best third-ranked sides.

The new format will lead to a total of 51 games, compared with the present 31, played over a period of 29 to 31 days depending on the match schedule.

In further decisions, UEFA is to establish a special unit to combat the threat of match-rigging by betting syndicates.

The UEFA Cup is to be renamed the UEFA Europa League with a new format from the 2009/10 season.

The competition would effectively be a new one with a 48-team group stage, centralised marketing of broadcast rights, a presenting sponsor, a new logo and an official match ball.

The decision to create a group stage with home and away matches had been taken by the executive committee in Lucerne, Switzerland in December.

The new format "will encourage teams from emerging countries or lesser-known teams to challenge the old order, and the new identity will seek to reflect that," UEFA said.

Platini said: "These changes will improve this historic competition which is very important for UEFA and for European football, as it gives more fans, players and clubs the thrill of European club football.

"I am convinced the new format will give the UEFA Europa League a successful new impetus."