BRUSSELS, Oct 30 (Reuters) - The European Union's Brexit negotiator Michel Barnier said the bloc will only give as much access to its single market to Britain after Brexit as is justified by London ensuring that EU rules and standards are preserved.

Barnier said access would be "proportional" and that British Prime Minister Boris Johnson may be able to achieve the goal of a zero-tariff and zero-quota new trade relationship.

"We want to have solid guarantees on the level-playing field," Barnier said. "We will keep a close watch and be extremely vigilant on ... social rights, environmental protection, state aid and obviously on issues of taxation."

"It will be a difficult and demanding set of negotiations," he said, adding that time would be extremely short to negotiate a new trade deal with Britain after Brexit and before the end of 2020.

He said that at the end of 2020 the sides would need to reassess whether more time was needed in the status-quo transition period envisaged after Brexit to agree on a new deal. (Reporting by Gabriela Baczynska; Editing by Hugh Lawson)