Boris Johnson has fuelled fears that workers’ rights and environmental safeguards will be ditched after Brexit after the government watered down a promise to enshrine them in law.

Downing Street suggested that the prime minister is no longer committed to pledges, made to MPs before the general election, to guarantee that standards will not be weakened when Britain leaves the EU.

A promise that MPs would be given a vote on whether or not to extend the transition period at the end of 2020 to avoid leaving the EU without a trade deal has also been ditched and will not be included in a key Brexit bill to be reintroduced this week.

The pledges were made by Mr Johnson and his team in October as they attempted to convince MPs to vote for the prime minister’s Brexit deal.

The bill implementing the deal passed its early stages in the Commons before Mr Johnson opted to pull the plug and instead call a general election. The subsequent poll delivered a governing majority of 80 for the Conservatives.

The government plans to reintroduce the withdrawal agreement bill, which implements Mr Johnson’s deal, this week. No 10 said it would be based on the deal that the prime minister secured with the EU, and would not necessarily include the guarantees made since then.

The original draft of the legislation included provisions to ensure that workers’ rights were not weakened after Brexit. Under questioning from MPs in October, Mr Johnson also promised to include a similar measure on environmental protections.

However, No 10 refused to say that the guarantees would be in the revised draft being brought back this week.

Mr Johnson’s spokesperson said: “We plan to start the process before Christmas and will do so in the proper constitutional way in consultation with the speaker.”

The spokesperson said the new version of the bill would reflect the agreement reached with Brussels in October, indicating that it will not necessarily feature any concessions made by ministers to MPs since then.

They said: “We will present a bill which will ensure we get Brexit done before the end of January. It will reflect the agreements made with the EU on our withdrawal.”

It is understood that work is still underway on the final wording of the new bill.

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Under the pledges made in October, ministers introducing new legislation after Brexit would have to guarantee that it would not weaken current EU employment and environmental standards. The government also vowed to look at strengthening unfair dismissal protections and the rights of workers after a business takeover.

Ministers would also have had to report regularly on new EU legislation on protections and whether the UK planned to voluntarily adopt the standards.

As part of the package of concessions designed to win MPs’ support, the government also promised to give the Commons a vote on any future trade deal with the EU. If no deal was in place by the end of next year, MPs would have been able to vote to seek an extension to the Brexit transition period.

Speaking in the Commons on 22 October, Robert Buckland told MPs: “What would happen if there were potential for an extension of the implementation period has been ​concerning members right across the House.