LONDON (Reuters) - Britain s main opposition Labour Party must commit to staying in the European Union s single market after Brexit and put an end to Prime Minister Theresa May s  destructive Brexit , several of its lawmakers said on Sunday. At the beginning of the party s annual conference, Labour lawmakers and trade union leaders wrote an open letter to their leader, calling on Jeremy Corbyn to  go further  than his current position to press for being part of the single market during any transition after Britain leaves in March 2019. After months of sticking to a position little different from the governing Conservative Party s pursuit of a clean break with the EU, Labour changed tack last month by saying, if in power, the party would press for full access to the single market during a transition - something that could continue beyond.  We have watched with dismay as the government has used a narrow referendum result to justify an extreme approach to Brexit,  the lawmakers said in a letter published in Britain s Observer newspaper.  So at our conference this week, Labour should commit to staying in the Single Market and Customs Union   ruling out no options for how to achieve this.  Labour s earlier move was aimed at increasing pressure on May, weakened after losing her party s majority in parliament in a June election she did not need to call and struggling to unite her own party over Brexit. But Labour is also keenly aware that many of its supporters, especially in northern and central England, voted for Brexit and want controls on immigration which many blame for stretching public services, such as hospitals and schools. Corbyn, who says he is preparing for government, told the BBC he was listening to those members of the Labour Party who want to keep the closest possible ties with the EU, but he did not say whether he would change the party s policy. Instead, he said he believed a transitional period with full single market access should last for as long as is necessary and offered some support for continued migration. In the letter, Labour lawmakers attacked the Conservative Party s  damaging promise to cut net migration to the tens of thousands .  Labour must be clear: migrants are not to blame for falling wages, job insecurity, bad housing and overstretched public services,  the letter said. 